xm_internal.c: remove misleading dead code

* src/xen/xm_internal.c (xenXMConfigGetULong): Remove useless and
misleading test (always false) for val->str == NULL before code that
always dereferences val->str.  "val" comes from virConfGetValue, and
at that point, val->str is guaranteed to be non-NULL.
(xenXMConfigGetBool): Likewise.
* src/util/conf.c (virConfSetValue): Ensure that vir->str is never NULL,
not even if someone tries to set such a value via virConfSetValue.

src/util/conf.c

@@ -858,6 +858,9 @@ virConfSetValue (virConfPtr conf,
 {
     virConfEntryPtr cur, prev = NULL;
 
+    if (value && value->type == VIR_CONF_STRING && value->str == NULL)
+        return -1;
+
     cur = conf->entries;
     while (cur != NULL) {
         if ((cur->name != NULL) && (STREQ(cur->name, setting))) {

src/xen/xm_internal.c

@@ -152,9 +152,6 @@ static int xenXMConfigGetBool(virConnectPtr conn,
     if (val->type == VIR_CONF_LONG) {
         *value = val->l ? 1 : 0;
     } else if (val->type == VIR_CONF_STRING) {
-        if (!val->str) {
-            *value = def;
-        }
         *value = STREQ(val->str, "1") ? 1 : 0;
     } else {
         xenXMError(conn, VIR_ERR_INTERNAL_ERROR,
@@ -183,9 +180,6 @@ static int xenXMConfigGetULong(virConnectPtr conn,
         *value = val->l;
     } else if (val->type == VIR_CONF_STRING) {
         char *ret;
-        if (!val->str) {
-            *value = def;
-        }
         *value = strtol(val->str, &ret, 10);
         if (ret == val->str) {
             xenXMError(conn, VIR_ERR_INTERNAL_ERROR,