[util] Avoid calling fclose(NULL) in zbin.c

Must check that argument to a fclose() is not NULL -- we can get to the
'err' label when file was not opened.  fclose(NULL) is known to produce
core dump on some platforms and we don't want zbin to fail so loudly.

Signed-off-by: Eygene Ryabinkin <rea-fbsd@codelabs.ru>

src/util/zbin.c

 	return 0;
 
  err:
-	fclose ( file );
+	if ( file )
+		fclose ( file );
 	return -1;
 }