[cogl/cogl-1.22] Fix compiler warning

[Emmanuele Bassi <ebassi src gnome org>]

commit b21de497d17d355b2515e82248590e6883fdc523
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Sep 23 13:56:15 2015 +0100

    Fix compiler warning
    
    Return a boolean value, not NULL.

 cogl/driver/gl/cogl-framebuffer-gl.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/cogl/driver/gl/cogl-framebuffer-gl.c b/cogl/driver/gl/cogl-framebuffer-gl.c
index 0fd6bc5..a30ccc1 100644
--- a/cogl/driver/gl/cogl-framebuffer-gl.c
+++ b/cogl/driver/gl/cogl-framebuffer-gl.c
@@ -810,7 +810,7 @@ _cogl_offscreen_gl_allocate (CoglOffscreen *offscreen,
 
   _COGL_RETURN_VAL_IF_FAIL (offscreen->texture_level <
                             _cogl_texture_get_n_levels (offscreen->texture),
-                            NULL);
+                            FALSE);
 
   _cogl_texture_get_level_size (offscreen->texture,
                                 offscreen->texture_level,