[gnome-builder] gdbwire: fix UB for parameter

[Christian Hergert <chergert src gnome org>]

commit be6c1f55757d74a6b4faef590fe1db97abb2ca32
Author: Christian Hergert <chergert redhat com>
Date:   Mon Jun 11 23:14:53 2018 -0700

    gdbwire: fix UB for parameter

 src/plugins/gdb/gdbwire.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/plugins/gdb/gdbwire.c b/src/plugins/gdb/gdbwire.c
index 5d87c268c..d8663f156 100644
--- a/src/plugins/gdb/gdbwire.c
+++ b/src/plugins/gdb/gdbwire.c
@@ -609,6 +609,9 @@ gdbwire_logger_log(const char *file, int line, enum gdbwire_logger_level level,
     char *buf;
     int size;
 
+    if (fmt == NULL)
+      return;
+
     va_list ap;
     va_start(ap, fmt);