avfilter/avfilter: add a return at the end of a non-void function

Fixes compilation with GCC 11 when configured with "--disable-optimizations --toolchain=gcc-tsan"

Signed-off-by: James Almer <jamrial@gmail.com>

libavfilter/avfilter.c

@@ -588,6 +588,7 @@ int avfilter_pad_count(const AVFilterPad *pads)
     }
 
     av_assert0(!"AVFilterPad list not from a filter");
+    return AVERROR_BUG;
 }
 #endif