jpeg: add jpeg_fetch_size()

This aids the fuzzer test case.

Change-Id: Ic7d43b76cf5660e085e7b3b13499de0358c13197
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/12181
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins)
2 files changed