util/error: make func optional

The function name is not available in Rust, so make it optional.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/include/qapi/error-internal.h b/include/qapi/error-internal.h
index 1ec3ceb..ff18a20 100644
--- a/include/qapi/error-internal.h
+++ b/include/qapi/error-internal.h
@@ -18,6 +18,8 @@
 {
     char *msg;
     ErrorClass err_class;
+
+    /* Used for error_abort only, may be NULL. */
     const char *func;
 
     /*
diff --git a/util/error.c b/util/error.c
index 3449ecc..daea214 100644
--- a/util/error.c
+++ b/util/error.c
@@ -24,8 +24,13 @@
 static void error_handle(Error **errp, Error *err)
 {
     if (errp == &error_abort) {
-        fprintf(stderr, "Unexpected error in %s() at %.*s:%d:\n",
-                err->func, err->src_len, err->src, err->line);
+        if (err->func) {
+            fprintf(stderr, "Unexpected error in %s() at %.*s:%d:\n",
+                    err->func, err->src_len, err->src, err->line);
+        } else {
+            fprintf(stderr, "Unexpected error at %.*s:%d:\n",
+		    err->src_len, err->src, err->line);
+        }
         error_report("%s", error_get_pretty(err));
         if (err->hint) {
             error_printf("%s", err->hint->str);