[gvfs] metadata: check for mmap failure

[Ondrej Holy <oholy src gnome org>]

commit ab2ced6131a90c857306374c99e69769141dc11c
Author: Tobias Mueller <muelli cryptobitch de>
Date:   Wed Jun 10 21:16:31 2020 +0200

    metadata: check for mmap failure
    
    mmap returns MAP_FAILED if, well, it failed. Thus, checking for
    NULL does not catch errors.

 metadata/metabuilder.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/metadata/metabuilder.c b/metadata/metabuilder.c
index f45d20ae..03063e68 100644
--- a/metadata/metabuilder.c
+++ b/metadata/metabuilder.c
@@ -1234,7 +1234,7 @@ meta_builder_write (MetaBuilder *builder,
 
       data = mmap (NULL, RANDOM_TAG_OFFSET + 4, PROT_READ|PROT_WRITE, MAP_SHARED, fd2, 0);
 
-      if (data)
+      if (data != MAP_FAILED)
        {
          old_tag = GUINT32_FROM_BE (*(guint32 *)(data + RANDOM_TAG_OFFSET));
          *(guint32 *)(data + ROTATED_OFFSET) = 0xffffffff;