lavc/libx264.c: Fix possible UB by NULL pointer LHS

It is UB to attempt to do pointer arithmetic on NULL pointer LHS, even if that pointer arithmetic ends up being "+= 0" (i.e. !!p == 0 if p == NULL).

libavcodec/libx264.c

@@ -940,7 +940,9 @@ static av_cold int X264_init(AVCodecContext *avctx)
                     return ret;
             }
             p= strchr(p, ':');
-            p+=!!p;
+            if (p) {
+                ++p;
+            }
         }
     }