storage: Resolve Coverity FORWARD_NULL

Coverity points out it's possible for one of the virCommand{Output|Error}*
API's to have not allocated 'output' and/or 'error' in which case the
strstr comparison will cause a NULL deref
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/storage/storage_backend_disk.c

@@ -413,8 +413,8 @@ virStorageBackendDiskFindLabel(const char* device)
     /* if parted succeeds we have a valid partition table */
     ret = virCommandRun(cmd, NULL);
     if (ret < 0) {
-        if (strstr(output, "unrecognised disk label") ||
-            strstr(error, "unrecognised disk label")) {
+        if ((output && strstr(output, "unrecognised disk label")) ||
+            (error && strstr(error, "unrecognised disk label"))) {
             ret = 1;
         }
     }