Fix some warnings under gcc 4.7.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/resources/imagewriter.cpp b/src/resources/imagewriter.cpp
index e6f3c8c27..2498f9c31 100644
--- a/src/resources/imagewriter.cpp
+++ b/src/resources/imagewriter.cpp
@@ -60,14 +60,14 @@ bool ImageWriter::writePNG(SDL_Surface *surface, const std::string &filename)
     info_ptr = png_create_info_struct(png_ptr);
     if (!info_ptr)
     {
-        png_destroy_write_struct(&png_ptr, static_cast<png_infopp>(NULL));
+        png_destroy_write_struct(&png_ptr, static_cast<png_infopp>(nullptr));
         logger->log1("Could not create png_info");
         return false;
     }
 
     if (setjmp(png_jmpbuf(png_ptr)))
     {
-        png_destroy_write_struct(&png_ptr, static_cast<png_infopp>(NULL));
+        png_destroy_write_struct(&png_ptr, static_cast<png_infopp>(nullptr));
         logger->log("problem writing to %s", filename.c_str());
         return false;
     }
@@ -113,7 +113,7 @@ bool ImageWriter::writePNG(SDL_Surface *surface, const std::string &filename)
 
     delete [] row_pointers;
 
-    png_destroy_write_struct(&png_ptr, static_cast<png_infopp>(NULL));
+    png_destroy_write_struct(&png_ptr, static_cast<png_infopp>(nullptr));
 
     if (SDL_MUSTLOCK(surface))
         SDL_UnlockSurface(surface);