vmnc: Fail if bpp is not recognized instead of crashing.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vmnc.c

@@ -484,6 +484,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
         break;
     default:
         av_log(avctx, AV_LOG_ERROR, "Unsupported bitdepth %i\n", c->bpp);
+        return AVERROR_PATCHWELCOME;
     }
 
     return 0;