libext2fs: avoid buffer overflow if s_first_meta_bg is too big

libext2fs: avoid buffer overflow if s_first_meta_bg is too big

If s_first_meta_bg is greater than the of number block group
descriptor blocks, then reading or writing the block group descriptors
will end up overruning the memory buffer allocated for the
descriptors.  Fix this by limiting first_meta_bg to no more than
fs->desc_blocks.  This doesn't correct the bad s_first_meta_bg value,
but it avoids causing the e2fsprogs userspace programs from
potentially crashing.

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