error: let error_is_type take a NULL error

Reported-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/error.c b/error.c
index b802752..68c0039 100644
--- a/error.c
+++ b/error.c
@@ -97,6 +97,10 @@
     char *ptr;
     char *end;
 
+    if (!err) {
+        return false;
+    }
+
     ptr = strstr(fmt, "'class': '");
     assert(ptr != NULL);
     ptr += strlen("'class': '");