Fix resource leak in initdb -X option

When creating the symlink for the xlog directory, free the string
which stores the link location.  Not really an issue but it doesn't
hurt to be good about this- prior cleanups have fixed similar
issues.

Leak found by the Coverity scanner.

Not back-patching as I don't see it being worth the code churn.

src/bin/initdb/initdb.c

@@ -3309,6 +3309,7 @@ create_xlog_symlink(void)
 		fprintf(stderr, _("%s: symlinks are not supported on this platform"));
 		exit_nicely();
 #endif
+		free(linkloc);
 	}
 }