ext2: Check for e2fs_first_dblock in ext2_compute_sb_data()

This prevents a kernel panic on a damaged ext2 superblock.

PR: 259107
Reported by: Robert Morris <rtm@lcs.mit.edu>
Differential Revision: https://reviews.freebsd.org/D33029

(cherry picked from commit 3dd3a395ba975d0fbe13320e6e69fb85b037da5e)