Fix memory leak when setting opaque on Frame

[lgeiger]

import resource
import av

def get_rss():
    return resource.getrusage(resource.RUSAGE_SELF).ru_maxrss

print(f"Base RSS memory: {get_rss():,} bytes")

print("Running 1,000,000 iterations of setting .opaque...")
for i in range(1_000_000):
    frame = av.VideoFrame(16, 16, "yuv420p")
    frame.opaque = i  # This triggers the av_malloc in opaque.py
    del frame

print(f"Final RSS memory: {get_rss():,} bytes")

main:

Base RSS memory: 42,450,944 bytes
Running 1,000,000 iterations of setting .opaque...
Final RSS memory: 57,950,208 bytes

This PR:

Base RSS memory: 42,287,104 bytes
Running 1,000,000 iterations of setting .opaque...
Final RSS memory: 42,287,104 bytes