opensc-explorer.c: fix compiler warning

opensc-explorer.c:1727:8: warning: comparison of integers of different signs:
      'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
                if(r > len) {
                   ~ ^ ~~~

src/tools/opensc-explorer.c

@@ -1724,7 +1724,7 @@ static int do_asn1(int argc, char **argv)
 		/* some cards return a bogus value for file length. As
 		 * long as the actual length is not higher than the expected
 		 * length, continue */
-		if(r > len) {
+		if(r > (signed)len) {
 			goto err;
 		}
 	}