tree-optimization/101031 - fix strlen opt invalidation logic

strlen opt uses ao_ref_init_from_ptr_and_size to prepare alias
queries to invalidate its knowledge about strings.  It constrains
the size using the number of known-nonzero chars and adds one
for a terminating nul - without knowing whether such nul exists
or even fits the object.  The latter is now a problem since the
oracle disambiguates an access of size two (as built so) against
a store to a plain char variable (where a terminating nul does not
fit).  The fix is to instead increment max_size but leave size to
the number of chars we know are accessed.

2021-06-14  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101031
	* tree-ssa-strlen.c (maybe_invalidate): Increment max_size
	instead of size when accounting for a possibly string
	terminating nul.

	* gcc.dg/torture/pr101031.c: New testcase.

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