[libglnx] errors: check for an error before prefixing

[Colin Walters <walters src gnome org>]

commit c820571bc4389515902e268bea5489dc114c3973
Author: Jonathan Lebon <jlebon redhat com>
Date:   Fri Jul 21 14:07:12 2017 -0700

    errors: check for an error before prefixing
    
    Minor tweak to the new `GLNX_AUTO_PREFIX_ERROR`. Since the common case
    is that there's no errors, let's bring down the same check that
    `g_prefix_error` does to avoid a function call most of the time.

 glnx-errors.h |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/glnx-errors.h b/glnx-errors.h
index 07c30fe..5a6fe19 100644
--- a/glnx-errors.h
+++ b/glnx-errors.h
@@ -94,7 +94,7 @@ glnx_prefix_error (GError **error, const char *fmt, ...)
  * ```
  * gboolean start_http_request (..., GError **error)
  * {
- *   GLNX_AUTO_PREFIX_ERROR("HTTP request", error)
+ *   GLNX_AUTO_PREFIX_ERROR ("HTTP request", error)
  *
  *   if (!libhttp_request_start (..., error))
  *     return FALSE;
@@ -109,7 +109,8 @@ typedef struct {
 static inline void
 glnx_cleanup_auto_prefix_error (GLnxAutoErrorPrefix *prefix)
 {
-  g_prefix_error (prefix->error, "%s: ", prefix->prefix);
+  if (prefix->error && *(prefix->error))
+    g_prefix_error (prefix->error, "%s: ", prefix->prefix);
 }
 G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(GLnxAutoErrorPrefix, glnx_cleanup_auto_prefix_error)
 #define GLNX_AUTO_PREFIX_ERROR(text, error) \