[gcab] Fix a few 'Dereference of null pointer' warnings

[Marc-Andre Lureau <malureau src gnome org>]

commit d61f2c76381fae21b79de11389595c82e2458f62
Author: Richard Hughes <richard hughsie com>
Date:   Wed Mar 1 16:39:27 2017 +0000

    Fix a few 'Dereference of null pointer' warnings
    
    https://bugzilla.gnome.org/show_bug.cgi?id=779462

 libgcab/cabinet.c      |    2 +-
 libgcab/gcab-cabinet.c |    2 +-
 libgcab/gcab-folder.c  |    2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/libgcab/cabinet.c b/libgcab/cabinet.c
index 1fa2af8..4df9024 100644
--- a/libgcab/cabinet.c
+++ b/libgcab/cabinet.c
@@ -562,7 +562,7 @@ end:
         g_set_error (error, GCAB_ERROR, GCAB_ERROR_FAILED,
                      "zlib failed: %s", zError (zret));
 
-    if (!*error && !success)
+    if (error != NULL && *error == NULL && !success)
         g_set_error (error, GCAB_ERROR, GCAB_ERROR_FAILED,
                      "Invalid cabinet chunk");
 
diff --git a/libgcab/gcab-cabinet.c b/libgcab/gcab-cabinet.c
index e81b052..a6cc4f0 100644
--- a/libgcab/gcab-cabinet.c
+++ b/libgcab/gcab-cabinet.c
@@ -257,7 +257,7 @@ gcab_cabinet_write (GCabCabinet *self,
 
         g_clear_object (&in);
         in = G_INPUT_STREAM (g_file_read (file->file, cancellable, error));
-        if (*error)
+        if (in == NULL)
             goto end;
 
         while ((len = g_input_stream_read (in,
diff --git a/libgcab/gcab-folder.c b/libgcab/gcab-folder.c
index e724097..dc33b6a 100644
--- a/libgcab/gcab-folder.c
+++ b/libgcab/gcab-folder.c
@@ -240,7 +240,7 @@ gcab_folder_add_file (GCabFolder *self, GCabFile *file,
         g_return_val_if_fail (G_IS_FILE (gfile), FALSE);
 
         GFileInfo *info = g_file_query_info (gfile, FILE_ATTRS, 0, NULL, error);
-        if (*error)
+        if (info == NULL)
             return FALSE;
 
         success = add_file_info (self, file, info,