h264: Fix assignments in if()

Fixes null pointer dereference later, since if this function failed,
a positive return value was returned to the caller.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/h264.c

@@ -2527,7 +2527,7 @@ static int h264_slice_header_init(H264Context *h, int reinit)
             return ret;
         }
     } else {
-        if ((ret = ff_MPV_common_init(s) < 0)) {
+        if ((ret = ff_MPV_common_init(s)) < 0) {
             av_log(h->s.avctx, AV_LOG_ERROR, "ff_MPV_common_init() failed.\n");
             return ret;
         }