libpng: add error handling to non existing file

libpng/img.c

@@ -7,6 +7,10 @@ image initialize_png(char *title, char *filename, int width, int height)
 	FILE *fp = NULL;
 
 	fp = fopen(filename, "wb");
+	if (fp == NULL) {
+		fprintf(stderr, "could not create %s", filename);
+		exit(EXIT_FAILURE);
+	}
 
 	png_structp png_ptr =
 	    png_create_write_struct(PNG_LIBPNG_VER_STRING,