Bugfix communication.cpp : guard against buffer[-1] when newline is first byte

[hubertciebiada]

Problem

software_stm32/src/communication.cpp:207 accesses buffer[c-1] without checking that c > 0:

for (unsigned int c = 0; c < buflen; c++) {
    if (buffer[c] == '\n') {
        if (buffer[c-1] == '\r') buffer[c-1] = '\0';
        ...
    }
}

When c == 0 and the first byte happens to be \n, buffer[c-1] reads buffer[-1] — out of bounds. This is undefined behaviour per the C/C++ standard.

In practice it is unlikely (the outer guard if (buflen >= 5) and the convention that ESP32 sends \r\n make a leading \n rare), but it can happen on UART glitches, partial frames, or right after reset. On STM32 this typically reads stack/heap memory that happens to sit just before buffer, so the comparison may spuriously succeed and the code then writes '\0' to that out-of-bounds location too — silent memory corruption.

Fix

Add c > 0 && to the condition. One-line defensive guard.