undefined behavior in hol_append()

[Matthew Wala]

Hello:

The function hol_append() in lib/argp-parse.c exhibits undefined behavior.
Specifically, the expression in line 894:

e->short_options += (short_options - hol->short_options);

subtracts two pointers that do not point to the same memory object.
Something like:

e->short_options = short_options + (e->short_options - hol->short_options);

does the intended operation with legal pointer arithmetic.

I found this issue with the help of SAFECode (http://safecode.cs.illinois.edu/).

Matt