gif2webp: fix transcode of loop count=65535

with loop_compatibility disabled (the default), non-zero loop counts
will be incremented by 1 for browser rendering compatibility. the max,
65535, is a special case as the muxer will fail if it is exceeded; avoid
increasing the limit in this case. this isn't 100% correct, but should
be close enough given the high number of iterations.

BUG=webp:382

Change-Id: Icde3e98a58e9ee89604a72fafda30ab71060dec5

examples/gif2webp.c

@@ -478,7 +478,7 @@ int main(int argc, const char *argv[]) {
         stored_loop_count = 1;
         loop_count = 1;
       }
-    } else if (loop_count > 0) {
+    } else if (loop_count > 0 && loop_count < 65535) {
       // adapt GIF's semantic to WebP's (except in the infinite-loop case)
       loop_count += 1;
     }