dbghelp: Return NULL instead of IMAGE_NO_MAP when PE file mapping fails.

A NULL test is always applied to the return value from pe_map_full(), but
IMAGE_NO_MAP is not NULL, it is ((void*)-1).

Signed-off-by: Conor McCarthy <cmccarthy@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/dbghelp/pe_module.c

@@ -53,7 +53,7 @@ static void* pe_map_full(struct image_file_map* fmap, IMAGE_NT_HEADERS** nth)
         fmap->u.pe.full_count++;
         return fmap->u.pe.full_map;
     }
-    return IMAGE_NO_MAP;
+    return NULL;
 }
 
 static void pe_unmap_full(struct image_file_map* fmap)