Fix varargs handling in col_{add,append}_lstr().

We do multiple va_start() calls using the first string in the list of
strings; do *not* use the first-string argument to iterate over all the
argument strings, as that means that only the first va_start() call will
do the right thing, use a separate variable.

Bug: 10755
Change-Id: Ic4a6c24f911e335d147883a25d30289628836875
Reviewed-on: https://code.wireshark.org/review/5630
Reviewed-by: Guy Harris <guy@alum.mit.edu>