resize2fs: Fix fix uninit group test accessing invalid memory

resize2fs: Fix fix uninit group test accessing invalid memory

Commit 74128f8 added tests for uninit groups, but it could access past
the end of the group_desc[] array after processing the last group:

==19668== Invalid read of size 2
==19668==    at 0x40518C: resize_fs (resize2fs.c:1824)
==19668==    by 0x405A46: main (main.c:451)
==19668==  Address 0x5a0d002 is not stack'd, malloc'd or (recently) free'd
==19668==
==19668== Invalid read of size 2
==19668==    at 0x405391: resize_fs (resize2fs.c:1864)
==19668==    by 0x405A46: main (main.c:451)
==19668==  Address 0x5a0d002 is not stack'd, malloc'd or (recently) free'd
==19668==

It was found by Eric Sandeen running the regression suite through
valgrind.

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