virConfSetValue: Simplify condition

There's no need for condition of the following form:

  if (str && STREQ(str, dst))

since we have STREQ_NULLABLE macro that handles NULL cases.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/util/virconf.c

@@ -884,7 +884,7 @@ virConfSetValue(virConfPtr conf,
 
     cur = conf->entries;
     while (cur != NULL) {
-        if ((cur->name != NULL) && (STREQ(cur->name, setting)))
+        if (STREQ_NULLABLE(cur->name, setting))
             break;
         prev = cur;
         cur = cur->next;