Fix crash on loading animated previews while using newer ffmpeg

0  0x00007ffff6ba7592 in avcodec_is_open () from /usr/lib64/libavcodec.so.58
1  0x00007ffff6a83c0d in avcodec_close () from /usr/lib64/libavcodec.so.58
2  0x00007ffff6fab5b9 in avcodec_free_context () from /usr/lib64/libavcodec.so.58
3  0x0000555555a52075 in MovieDecoder_FFMpeg::Init (this=this@entry=0x55555695aae0) at <path>/stepmania/src/arch/MovieTexture/MovieTexture_FFMpeg.cpp:167
4  0x0000555555a528cc in MovieDecoder_FFMpeg::MovieDecoder_FFMpeg (this=0x55555695aae0) at <path>/stepmania/src/arch/MovieTexture/MovieTexture_FFMpeg.cpp:120
5  MovieTexture_FFMpeg::MovieTexture_FFMpeg (this=0x55555bd0b7f0, ID=...) at <path>/stepmania/src/arch/MovieTexture/MovieTexture_FFMpeg.cpp:564
6  0x0000555555a52a40 in RageMovieTextureDriver_FFMpeg::Create (this=this@entry=0x55555c5b8b60, ID=..., sError=...)
   at <path>/stepmania/src/arch/MovieTexture/MovieTexture_FFMpeg.cpp:570
7  0x0000555555a4ece3 in RageMovieTexture::Create (ID=...) at <path>/stepmania/src/arch/MovieTexture/MovieTexture.cpp:113
8  0x0000555555c32753 in RageTextureManager::LoadTextureInternal (this=<optimized out>, ID=...) at <path>/stepmania/src/RageTextureManager.cpp:168
9  0x0000555555c32951 in RageTextureManager::LoadTexture (this=this@entry=0x55555a9aa8f0, ID=...) at <path>/stepmania/src/RageTextureManager.cpp:184
10 0x000055555595f9e7 in Sprite::LoadFromTexture (this=0x55555b461900, ID=...) at <path>/stepmania/src/Sprite.cpp:362
11 0x000055555595fb7a in Sprite::Load (this=0x55555b461900, ID=...) at <path>/stepmania/src/Sprite.cpp:176
12 0x00005555559623d2 in LunaSprite::Load (p=0x55555b461900, L=0x5555567f9b50) at <path>/stepmania/src/Sprite.cpp:1132

src/arch/MovieTexture/MovieTexture_FFMpeg.cpp

@@ -112,6 +112,7 @@ MovieDecoder_FFMpeg::MovieDecoder_FFMpeg()
 	m_swsctx = NULL;
 	m_avioContext = NULL;
 	m_buffer = NULL;
+	m_pStreamCodec = NULL;
 	m_fctx = nullptr;
 	m_pStream = nullptr;
 	m_iCurrentPacketOffset = -1;
@@ -166,6 +167,7 @@ void MovieDecoder_FFMpeg::Init()
 	{
 		avcodec::avcodec_free_context(&m_pStreamCodec);
 	}
+	m_pStreamCodec = NULL;
 #endif
 }