Free the allocated rif_t image in rif.h

rif.h

@@ -23,4 +23,5 @@ void write_rif_little(char* path, uint32_t width, uint32_t size, uint8_t format,
   memcpy(((char*)img)+17, data, size);
   FILE* output_file = fopen(path, "w");
   fwrite(img, 1, IMG_SIZE, output_file);
+  free(img);
 }