Silence GCC warnings:

ibswscale/swscale.c: In function ‘sws_scale’:
libswscale/swscale.c:2678: warning: ‘b’ may be used uninitialized in
this function
libswscale/swscale.c:2678: warning: ‘g’ may be used uninitialized in
this function
libswscale/swscale.c:2678: warning: ‘r’ may be used uninitialized in
this function

Originally committed as revision 27837 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale
