Add array bounds checking in h_Enumerate

Add array bounds checking in h_Enumerate

When hostList is not properly NULL-terminated, the current code does
not protect from buffer overflow.  The following patch prevents buffer
overflow, prints a message, and asserts.

On our Linux hosts, we never reached the original assert, as there is
a problem handling the segfault the buffer overflow causes.

FIXES 125506

Change-Id: Ifce92c593d17050e45add9e37a7a9ed4fbc377ef
Reviewed-on: http://gerrit.openafs.org/757
Reviewed-by: Dan Hyde <drh@umich.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>