util: Alter virAuthGet*Path API to check valid callback

Before trying to call @auth->cb, let's ensure it exists.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NMarcos Paulo de Souza <marcos.souza.org@gmail.com>

src/util/virauth.c

@@ -175,6 +175,12 @@ virAuthGetUsernamePath(const char *path,
         if (auth->credtype[ncred] != VIR_CRED_AUTHNAME)
             continue;
 
+        if (!auth->cb) {
+            virReportError(VIR_ERR_INVALID_ARG, "%s",
+                           _("Missing authentication callback"));
+            return NULL;
+        }
+
         cred.type = VIR_CRED_AUTHNAME;
         cred.prompt = prompt;
         cred.challenge = hostname;
@@ -251,6 +257,12 @@ virAuthGetPasswordPath(const char *path,
             continue;
         }
 
+        if (!auth->cb) {
+            virReportError(VIR_ERR_INVALID_ARG, "%s",
+                           _("Missing authentication callback"));
+            return NULL;
+        }
+
         cred.type = auth->credtype[ncred];
         cred.prompt = prompt;
         cred.challenge = hostname;