fix: check return values of malloc in xbm2fb and realloc in cfg_desc_append

[Rampage270906]

Two unchecked heap allocations fixed:

1. xbm2fb() in xbm.c
   malloc return value was not checked. If allocation fails, the subsequent memset dereferences NULL. xbm2fb() is called every
   frame in the animation loop. Fixed by switching to calloc (which zeroes memory, removing the need for memset) and returning early if allocation fails.

2. cfg_desc_append() in usb/setup.c
   realloc return value was not checked, with a TODO comment left by the original author acknowledging the missing check. If realloc fails, the original cfg_desc pointer is overwritten with NULL(memory leak) and the subsequent memcpy dereferences NULL. Fixed by storing the result in a temporary pointer, checking for NULL, and only updating cfg_desc on success.

Summary by Sourcery

Ensure heap allocations in graphics and USB configuration code safely handle allocation failures.

Bug Fixes:

• Handle potential allocation failure in xbm2fb temporary framebuffer allocation to avoid NULL dereference in the animation loop.
• Guard cfg_desc_append against realloc failure to prevent losing the original buffer and dereferencing a NULL pointer.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds proper heap allocation failure handling in xbm2fb() and cfg_desc_append() by checking calloc/realloc results before use and avoiding NULL dereferences or leaks.

Flow diagram for updated xbm2fb heap allocation handling

graph TD
    A[xbm2fb entry] --> B[Compute W using ALIGN_8BIT]
    B --> C[Call calloc for tmpfb with W elements of uint16_t]
    C --> D{tmpfb is NULL?}
    D -- Yes --> E[Return early from xbm2fb]
    D -- No --> F{xbm height and row plus height are nonnegative and col nonnegative?}
    F -- No --> G[Function continues without rendering loop]
    F -- Yes --> H[Loop over h from 0 to xbm.h]
    H --> I[Loop over w from 0 to W]
    I --> J[Update tmpfb index w based on xbm bits]
    J --> K[Copy tmpfb row into framebuffer fb]
    K --> L[Advance framebuffer pointer and repeat loops]
    G --> M[xbm2fb exit]
    L --> M[xbm2fb exit]

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Flow diagram for updated cfg_desc_append realloc handling

graph TD
    A[cfg_desc_append entry] --> B[If cfg_desc is NULL set cfg_len from 0 else read wTotalLength]
    B --> C[Compute newlen as cfg_len plus len]
    C --> D[Call realloc on cfg_desc into tmp]
    D --> E{tmp is NULL?}
    E -- Yes --> F[Return early from cfg_desc_append without modifying cfg_desc]
    E -- No --> G[Assign cfg_desc from tmp]
    G --> H[Copy len bytes from desc into cfg_desc offset cfg_len]
    H --> I[cfg_desc_append exit]

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File-Level Changes

Change	Details	Files
Harden xbm2fb() against allocation failure and simplify initialization of the temporary framebuffer buffer.	Replace malloc+memset with calloc when allocating the temporary framebuffer line buffer Check the allocation result and return early if the allocation fails to avoid NULL dereference Use sizeof(*tmpfb) instead of hardcoding the element type size for the allocation	src/xbm.c
Make cfg_desc_append() safely handle realloc failures and avoid losing the original buffer pointer.	Store realloc result in a temporary pointer before updating the global buffer pointer Check the temporary pointer for NULL and return early if reallocation fails Only update cfg_desc on successful reallocation to avoid memory leaks and NULL dereferences	src/usb/setup.c

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[sourcery-ai]

Hey - I've left some high level feedback:

• Both new early returns on allocation failure silently do nothing; consider propagating an error (or at least a status indication) so callers can detect and respond to out-of-memory conditions instead of proceeding as if the operations succeeded.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Both new early returns on allocation failure silently do nothing; consider propagating an error (or at least a status indication) so callers can detect and respond to out-of-memory conditions instead of proceeding as if the operations succeeded.

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[CloudyPadmal]

Why is this removed?

[Rampage270906]

Sorry about that, accidentally removed it while editing. Restored now.