qemu: Resolve Coverity FORWARD_NULL

If the virJSONValueNewObject() fails, then rather than going to error
and getting a Coverity false positive since it doesn't seem to understand
the relationship between nkeywords, keywords, and values and seems to
believe calling qemuFreeKeywords will cause a NULL deref - just return NULL
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_monitor_json.c

@@ -639,7 +639,7 @@ qemuMonitorJSONKeywordStringToJSON(const char *str, const char *firstkeyword)
     size_t i;
 
     if (!(ret = virJSONValueNewObject()))
-        goto error;
+        return NULL;
 
     if (qemuParseKeywords(str, &keywords, &values, &nkeywords, 1) < 0)
         goto error;