Plug a memory leak in tre_match().

When not using `alloca()`, we need to free `tags` before returning.

Obtained from the R repository.

lib/regexec.c

@@ -163,6 +163,10 @@ tre_match(const tre_tnfa_t *tnfa, const void *string, size_t len,
 	  if (source->rewind == NULL || source->compare == NULL)
 	    /* The backtracking matcher requires rewind and compare
 	       capabilities from the input stream. */
+#ifndef TRE_USE_ALLOCA
+	    if (tags)
+	      xfree(tags);
+#endif /* !TRE_USE_ALLOCA */
 	    return REG_BADPAT;
 	}
       status = tre_tnfa_run_backtrack(tnfa, string, (int)len, type,