string-list: report programming error with BUG

string-list: report programming error with BUG

Passing a string list that has .strdup_strings bit unset to
string_list_split(), or one that has .strdup_strings bit set to
string_list_split_in_place(), is a programmer error.  Do not use
die() to abort the execution.  Use BUG() instead.

As a developer-facing message, the message string itself should
be a lot more concise, but let's keep the original one for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>