mtar_seek changes position even if seek callbak failed

[StellaSmith]

microtar/src/microtar.c

Lines 216 to 220 in 27076e1

	int mtar_seek(mtar_t *tar, unsigned pos) {
	int err = tar->seek(tar, pos);
	tar->pos = pos;
	return err;
	}

The position is updated regardless if the seek operation fails.
As for fseek, the C standard specifies that if an error occurs, the file position is unchanged.