engine: imagelib: img_png: fix Mem_Free on null ptr

engine/common/imagelib/img_png.c

@@ -214,7 +214,7 @@ qboolean Image_LoadPNG( const char *name, const byte *buffer, fs_offset_t filesi
 		if( ntohl( crc32 ) != crc32_check )
 		{
 			Con_DPrintf( S_ERROR "Image_LoadPNG: Found chunk with wrong CRC32 sum (%s)\n", name );
-			Mem_Free( idat_buf );
+			if( idat_buf ) Mem_Free( idat_buf );
 			return false;
 		}
 
@@ -222,6 +222,12 @@ qboolean Image_LoadPNG( const char *name, const byte *buffer, fs_offset_t filesi
 		buf_p += sizeof( crc32 );
 	}
 
+	if( oldsize == 0 )
+	{
+		Con_DPrintf( S_ERROR "Image_LoadPNG: Couldn't find IDAT chunks (%s)\n", name );
+		return false;
+	}
+
 	if( png_hdr.ihdr_chunk.colortype == PNG_CT_PALLETE && !pallete )
 	{
 		Con_DPrintf( S_ERROR "Image_LoadPNG: PLTE chunk not found (%s)\n", name );
@@ -243,12 +249,6 @@ qboolean Image_LoadPNG( const char *name, const byte *buffer, fs_offset_t filesi
 		return false;
 	}
 
-	if( oldsize == 0 )
-	{
-		Con_DPrintf( S_ERROR "Image_LoadPNG: Couldn't find IDAT chunks (%s)\n", name );
-		return false;
-	}
-
 	switch( png_hdr.ihdr_chunk.colortype )
 	{
 	case PNG_CT_GREY: