apply: make update_pre_post_images() sanity check the given postlen

"git apply --whitespace=fix" used to be able to assume that fixing
errors will always reduce the size by e.g. stripping whitespaces at
the end of lines or collapsing runs of spaces into tabs at the
beginning of lines.  An update to accomodate fixes that lengthens
the result by e.g. expanding leading tabs into spaces were made long
time ago but the logic miscounted the necessary space after such
whitespace fixes, leading to either under-allocation or over-usage
of already allocated space.

Illustrate this with a runtime sanity-check to protect us from
future breakage.  The test was stolen from Kyle McKay who helped
to identify the problem.

Helped-by: "Kyle J. McKay" <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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