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>

include/qapi/error-internal.h

@@ -18,6 +18,8 @@ struct Error
 {
     char *msg;
     ErrorClass err_class;
+
+    /* Used for error_abort only, may be NULL. */
     const char *func;
 
     /*

util/error.c

@@ -24,8 +24,13 @@ Error *error_warn;
 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);