mkclean: initialize MetaSeekBefore

It's always initialized in non-live mode but MSVC complains.

mkclean/mkclean.c

@@ -1409,7 +1409,7 @@ int main(int argc, const char *argv[])
     ebml_parser_context RContext;
     ebml_parser_context RSegmentContext;
     int UpperElement;
-    filepos_t MetaSeekBefore, MetaSeekAfter;
+    filepos_t MetaSeekBefore = 0, MetaSeekAfter;
     filepos_t NextPos = 0, SegmentSize = 0, ClusterSize, CuesSize = MIN_FILEPOS;
     size_t ExtraVoidSize = 0;
     mkv_timestamp_t PrevTimestamp;