Fix building with C++14, which errors out due to bool -> T* conversions
See also: https://bugs.gentoo.org/show_bug.cgi?id=594052

--- a/src/FbTk/Image.cc
+++ b/src/FbTk/Image.cc
@@ -85,14 +85,14 @@
 
 
     if (filename == "")
-        return false;
+        return NULL;
 
     // determine file ending
     std::string extension(StringUtil::toUpper(StringUtil::findExtension(filename)));
     
     // valid handle?
     if (s_image_map.find(extension) == s_image_map.end())
-        return false;
+        return NULL;
     
     // load file
     PixmapWithMask *pm = s_image_map[extension]->load(filename, screen_num);