Add av_uninit macro to variables to avoid false positive warnings:

Add av_uninit macro to variables to avoid false positive warnings:
libavcodec/msrledec.c: In function ‘ff_msrle_decode’:
libavcodec/msrledec.c:139: warning: ‘pix32’ may be used uninitialized in this function
libavcodec/msrledec.c:138: warning: ‘pix16’ may be used uninitialized in this function

Originally committed as revision 17156 to svn://svn.ffmpeg.org/ffmpeg/trunk