phoenix_extract.py: correct filelength check

The script was checking the length after accessing the data.

Change-Id: I05ac3a2e3025b7a2a2369f9a7c9a34dfc60e9572
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-on: https://review.coreboot.org/14166
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed