fix(cobs): avoid out-of-bounds read in apex_cobs_decode_framed on all-zero input

[steps-re]

Summary

apex_cobs_decode_framed() reads one byte past the end of input when the buffer is entirely 0x00.

Details

The leading-delimiter skip loop dereferences input[i] before the bound check:

while (input[i] == 0x00 && i < length) i++;

&& evaluates left-to-right, so on an all-zero buffer i walks up to length and input[length] is read before the loop terminates. An all-zeros buffer is a realistic input on a UART wire (idle line held low, electrical noise, or a run of the 0x00 bytes APEX uses as frame delimiters), and this is a public API (apex_cobs.h) consuming bytes straight off the wire, so the out-of-bounds read is reachable from malformed/adversarial input.

Fix

Swap the operands so the bound is checked first. No behavior change on any valid input.

Verification

• Builds clean with the repo Makefile (-std=c99 -Wall -Wextra -Wpedantic), no new warnings.
• AddressSanitizer reproducer (heap buffer of exactly length zero bytes): before → heap-buffer-overflow READ of size 1 in the skip loop; after → no error, returns APEX_ERR_MALFORMED.
• Round-trip regression (apex_cobs_encode_framed → apex_cobs_decode_framed) under ASan still passes.

The sibling trailing-delimiter loop is already safe (bounded below by i). Note: test_cobs.cc doesn't currently exercise the _framed helpers — happy to add a DecodeFramedAllZeros case if you point me at the gtest vendor path.