Fix Memory leaks in FFmpegReader, Clip, and FrameMapper

[jonoomph]

Due to a recent regression, and some investigating into memory leaks (thanks to Valgrind), we found and fixed 3 serious memory leaks in libopenshot.

• AVFRAME in FFmpegReader::ProcessVideoPacket - initializing struct, but not deleting it if the packet was not ready
• AVFRAME in FFmpegReader::ProcessAudioPacket - initializing struct, but not deleting it if the packet was not ready
• Clip::final_cache was not being initialized correctly, and was defaulting to infinite Clip cache - always growing as large as the # of frames in the clip - oops - large leak

[jonoomph]

Memory Leak caused by Regression in Time Re-mapping

[codecov]

Codecov Report

Merging #909 (59e0862) into develop (ec0c0b1) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #909      +/-   ##
===========================================
+ Coverage    54.03%   54.09%   +0.05%     
===========================================
  Files          191      191              
  Lines        16842    16845       +3     
===========================================
+ Hits          9101     9112      +11     
+ Misses        7741     7733       -8     

Impacted Files	Coverage Δ
src/Clip.cpp	69.82% <100.00%> (ø)
src/FFmpegReader.cpp	74.47% <100.00%> (+0.04%)	⬆️
src/FrameMapper.cpp	92.17% <100.00%> (+1.36%)	⬆️

... and 1 file with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more