virconf: Resolve a possible memory leak in virConfSetValue

Found this one by inspection... The API claims to "own" the input
value even in the case of error.  However, in the initial entry
to the API if the value exists, was STRING, but without a str value
it just returned without freeing the 'value' which it claims to now
own.  So I added the virConfFreeValue() call in order to resolve.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/util/virconf.c

@@ -887,8 +887,10 @@ virConfSetValue(virConfPtr conf,
 {
     virConfEntryPtr cur, prev = NULL;
 
-    if (value && value->type == VIR_CONF_STRING && value->str == NULL)
+    if (value && value->type == VIR_CONF_STRING && value->str == NULL) {
+        virConfFreeValue(value);
         return -1;
+    }
 
     cur = conf->entries;
     while (cur != NULL) {