phyp: avoid a crash

This has been present since the introduction of phypAttachDevice
in commit 444fd07a.

* src/phyp/phyp_driver.c (phypAttachDevice): Don't dereference
NULL.

src/phyp/phyp_driver.c

@@ -1720,6 +1720,11 @@ phypAttachDevice(virDomainPtr domain, const char *xml)
     virBuffer buf = VIR_BUFFER_INITIALIZER;
     char *domain_name = NULL;
 
+    if (VIR_ALLOC(def) < 0) {
+        virReportOOMError();
+        goto cleanup;
+    }
+
     domain_name = escape_specialcharacters(domain->name);
 
     if (domain_name == NULL) {