e2fsck: Fix segmentation fault when checking a file system

If a corrupted file system causes us to want to delete an extent, and
that causes us to want to release a block in e2fsck pass #1, it would
be preferable if e2fsck didn't seg fault.  This tends to get users
craky, as users are wont to do.  :-)

Thanks to Dirk Reiners for reporting this bug:

e2fsck crashes fixing a corrupted 3.5 TB filesystem:

0x0000000000432002 in ext2fs_unmark_generic_bitmap (bitmap=0x0, bitno=623386749)
at gen_bitmap.c:183
183             if ((bitno < bitmap->start) || (bitno > bitmap->end)) {
(gdb) bt
bitno=623386749) at gen_bitmap.c:183
block=623386749) at ../../lib/ext2fs/bitops.h:319
inuse=-1) at alloc_stats.c:78
extent.c:1509
pb=0x7fffffffdfe0, start_block=0, ehandle=0x6dcf50) at pass1.c:1709
pb=0x7fffffffdfe0, start_block=0, ehandle=0x6dcf50) at pass1.c:1737
pctx=0x7fffffffe100, pb=0x7fffffffdfe0) at pass1.c:1842
block_buf=0x6c4330 "\373\212#") at pass1.c:1920

The source of the NULL bitmap is fs on stack frame 2:

(gdb) up 2
inuse=-1) at alloc_stats.c:78
78                      ext2fs_unmark_block_bitmap(fs->block_map, blk);

Addresses-SourceForge-Bug: #2971800

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

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