x11/fbdesk: Fix build with Clang 6

Image.cc:88:16: error: cannot initialize return object of type 'FbTk::PixmapWithMask *' with an rvalue of type 'bool'
        return false;
               ^~~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/fbdesk-1.4.1_6.log

x11/fbdesk/files/patch-src_FbTk_Image.cc

@@ -0,0 +1,19 @@
+--- src/FbTk/Image.cc.orig	2018-08-28 13:45:07 UTC
++++ src/FbTk/Image.cc
+@@ -85,14 +85,14 @@ PixmapWithMask *Image::load(const std::string &filenam
+ 
+ 
+     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);