Tests fail (maybe related to ffmpeg 3.4)

[marillat]

Git 0ca3f2d build with packages from www.deb-multimedia.org repository.
Recent changes : ffmpeg 3.4 and python 3.6

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     RUNNING ALL TESTS
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/home/marillat/libopenshot-master/tests/Clip_Tests.cpp:95:1: error: Failure in Clip_Properties: Unhandled exception: basic_string::_M_construct null not valid
[AudioData @ 0x7faa297f9750] invalid NULL pointer for src[0]
[AudioData @ 0x7faa2affc750] invalid NULL pointer for src[0]
[AudioData @ 0x7faa287f7750] invalid NULL pointer for src[0]
[AudioData @ 0x7faa2a7fb750] invalid NULL pointer for src[0]
[AudioData @ 0x7faa27ff6750] invalid NULL pointer for src[0]
[AudioData @ 0x7faa28ff8750] invalid NULL pointer for src[0]
[AudioData @ 0x7faa2a7fb750] invalid NULL pointer for src[0]
[libvpx @ 0x6bf745aa00] v1.6.1
[webm @ 0x6bf7459fa0] Using AVStream.codec.time_base as a timebase hint to the muxer is deprecated. Set AVStream.time_base instead.
[webm @ 0x6bf7459fa0] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
[webm @ 0x6bf7459fa0] Using AVStream.codec.time_base as a timebase hint to the muxer is deprecated. Set AVStream.time_base instead.
[webm @ 0x6bf7459fa0] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
[AudioData @ 0x7faa2affc750] invalid NULL pointer for src[0]
[AudioData @ 0x7faa2affc750] invalid NULL pointer for src[0]
/home/marillat/libopenshot-master/tests/FFmpegWriter_Tests.cpp:76:1: error: Failure in FFmpegWriter_Test_Webm: Expected 23 but was 11
/home/marillat/libopenshot-master/tests/FFmpegWriter_Tests.cpp:77:1: error: Failure in FFmpegWriter_Test_Webm: Expected 23 but was 11
/home/marillat/libopenshot-master/tests/FFmpegWriter_Tests.cpp:78:1: error: Failure in FFmpegWriter_Test_Webm: Expected 23 but was 11
[AudioData @ 0x7faa297f9750] invalid NULL pointer for src[0]
[AudioData @ 0x7ffdc3429600] invalid NULL pointer for src[0]
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
FAILURE: 2 out of 79 tests failed (4 failures).
Test time: 10.60 seconds.
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[ferdnyc]

Same as #69, please file a new report if this is still an issue.