util: plug memory leak

Leak detected by Coverity; only possible on unlikely ptsname_r
failure.  Additionally, the man page for ptsname_r states that
failure is merely non-zero, not necessarily -1.

* src/util/util.c (virFileOpenTtyAt): Avoid leak on ptsname_r
failure.

src/util/util.c

@@ -1126,9 +1126,11 @@ int virFileOpenTtyAt(const char *ptmx,
             goto cleanup;
         }
 
-        if (ptsname_r(*ttymaster, *ttyName, PATH_MAX) < 0)
+        if (ptsname_r(*ttymaster, *ttyName, PATH_MAX) != 0) {
+            VIR_FREE(*ttyName);
             goto cleanup;
         }
+    }
 
     rc = 0;