qemu: Don't return memory device config on error in qemuBuildMemoryBackendStr

In the last section if the function determines that the config is
invalid when QEMU doesn't support the memory device the JSON config
object would be returned even if it doesn't make sense.

Assign the object to be returned only on success.

src/qemu/qemu_command.c

@@ -4750,9 +4750,6 @@ qemuBuildMemoryBackendStr(unsigned long long size,
             goto cleanup;
     }
 
-    *backendProps = props;
-    props = NULL;
-
     if (!hugepage) {
         bool nodeSpecified = virDomainNumatuneNodeSpecified(def->numa, guestNode);
 
@@ -4767,11 +4764,15 @@ qemuBuildMemoryBackendStr(unsigned long long size,
         /* report back that using the new backend is not necessary to achieve
          * the desired configuration */
         if (!userNodeset && !nodeSpecified) {
+            *backendProps = props;
+            props = NULL;
             ret = 1;
             goto cleanup;
         }
     }
 
+    *backendProps = props;
+    props = NULL;
     ret = 0;
 
  cleanup: