bug #1500 fix inconsistent response from twig_slice (rodnaph)

This PR was squashed before being merged into the 1.16-dev branch (closes #1500).

Discussion
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fix inconsistent response from twig_slice

twig_slice can return a boolean when slicing a string outside of its bounds and mbstring is not available.  this would cause a knock-on error in twig_first/twig_last when calling current()

this (as suggested) adds a cast to emulate the behaviour of mb_substr for substr.

Commits
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c740060 fix inconsistent response from twig_slice