[PATCH] fix build with libpng 1.4.x

In 1.4.x the png_*_NULL defines are gone. Replace them with a normal
NULL instead.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
---
 src/SDL/i_sshot.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: prboom-2.5.0/src/SDL/i_sshot.c
===================================================================
--- prboom-2.5.0.orig/src/SDL/i_sshot.c
+++ prboom-2.5.0/src/SDL/i_sshot.c
@@ -231,7 +231,7 @@ int I_ScreenShot (const char *fname)
   if (fp)
   {
     png_struct *png_ptr = png_create_write_struct(
-        PNG_LIBPNG_VER_STRING, png_error_ptr_NULL, error_fn, warning_fn);
+        PNG_LIBPNG_VER_STRING, NULL, error_fn, warning_fn);
 
     if (png_ptr)
     {
@@ -279,7 +279,7 @@ int I_ScreenShot (const char *fname)
             break;
         }
       }
-      png_destroy_write_struct(&png_ptr,  png_infopp_NULL);
+      png_destroy_write_struct(&png_ptr, NULL);
     }
     fclose(fp);
   }