Use existing png_set_IHDR() and stop accessing PNG structure members directly

--- src/zip/zpng.c
+++ src/zip/zpng.c
@@ -129,7 +129,6 @@
         png_set_IHDR(png_ptr, info_ptr, width, height, 8,
                      PNG_COLOR_TYPE_RGB, PNG_INTERLACE_NONE,
                      PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
-        info_ptr->color_type = PNG_COLOR_TYPE_RGB;
 
         //Allocate an array of scanline pointers
         row_pointers = (png_bytep*)malloc(height*sizeof(png_bytep));