avresample: Make sure the even check does not overflow

CC: libav-stable@libav.org
Bug-Id: CID 732225

libavresample/audio_mix_matrix.c

@@ -60,7 +60,7 @@
 
 static av_always_inline int even(uint64_t layout)
 {
-    return (!layout || (layout & (layout - 1)));
+    return (!layout || !!(layout & (layout - 1)));
 }
 
 static int sane_layout(uint64_t layout)