Avoid conversion warning in minigzip.c.

Author: Mark Adler

test/minigzip.c

@@ -500,7 +500,7 @@ void file_uncompress(file)
     char *infile, *outfile;
     FILE  *out;
     gzFile in;
-    unsigned len = strlen(file);
+    z_size_t len = strlen(file);
 
     if (len + strlen(GZ_SUFFIX) >= sizeof(buf)) {
         fprintf(stderr, "%s: filename too long\n", prog);