Fix SIGSEGV when can't writer output

This fixes SIGSEGV (Segmentation Fault) when output file can't be opened for writting binary (wb). Instead display error message on what's wrong and exit with failure status (aka. errorlevel). This will make it more clear to user who forgot to "sudo" etc.

src/zopfli/zopfli_bin.c

@@ -71,6 +71,10 @@ Saves a file from a memory array, overwriting the file if it existed.
 static void SaveFile(const char* filename,
                      const unsigned char* in, size_t insize) {
   FILE* file = fopen(filename, "wb" );
+  if (file == NULL) {
+      fprintf(stderr,"Error: Cannot write to output file, terminating.\n");
+      exit (EXIT_FAILURE);
+  }
   assert(file);
   fwrite((char*)in, 1, insize, file);
   fclose(file);