Root Cause Analysis: Looking at the given vulnerable code line and its context, the CWE-787 (out-of-bounds write) vulnerability exists in the code because the `size` variable is calculated using `RAW_SAMPLES` and `s->streams[0]->codec->block_align` without any bounds checking. If the values of `RAW_SAMPLES` or `s->streams[0]->codec->block_align` are large or malicious, the calculated `size` can be larger than the actual buffer size allocated for `pkt`. This can lead to an out-of-bounds write when `av_get_packet` tries to write data beyond the allocated buffer size, resulting in a buffer overflow vulnerability, which is CWE-787.

However, without the definition of the `av_get_packet` function, it is difficult to determine the exact root cause of the vulnerability. Please provide the summary of the function `av_get_packet`.