shallow.c: avoid theoretical pointer wrap-around

shallow.c: avoid theoretical pointer wrap-around

The expression info->free+size is technically undefined behaviour in
exactly the case we want to test for. Moreover, the compiler is likely
to translate the expression to

  (unsigned long)info->free + size > (unsigned long)info->end

where there's at least a theoretical chance that the LHS could wrap
around 0, giving a false negative.

This might as well be written using pointer subtraction avoiding these
issues.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>