maint: avoid a static analysis warning in csplit

maint: avoid a static analysis warning in csplit

The Canalyze static code analyzer correctly surmised
that there is a use-after-free bug in free_buffer()
at the line "struct line *n = l->next", if that
function is called multiple times.

This is not a runtime issue since a list of lines
will not be present in the !lines_found case.

* src/csplit.c (free_buffer): Set list head to NULL so
that this function can be called multiple times.
(load_buffer): Remove a redundant call to free_buffer().

Reported-by: Xu Zhongxing