fix a problem with thread safety

gzvscanf() was using global static data for a buffer.  this patch moves it onto the stack

src/metaio.c

@@ -72,7 +72,7 @@
 static int gzvscanf( gzFile file, const char *fmt, va_list ap )
 {
     static const char terminators[] = {',', '\n', '\\', '\"', '\0'};
-    static char buf[4096];
+    char buf[4096];
     size_t i;
     for ( i = 0; i < sizeof(buf); i++ ) {
         int c = gzgetc( file );