Fix unchecked use of tree_to_uhwi in tree-ssa-strlen.c

Fix unchecked use of tree_to_uhwi in tree-ssa-strlen.c

r273783 introduced an unchecked use of tree_to_uhwi.  This is
tested by the SVE ACLE patches, but could potentially trigger
in non-SVE cases too.

2019-10-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-ssa-strlen.c (count_nonzero_bytes): Check tree_fits_uhwi_p
	before using tree_to_uhwi.

From-SVN: r276990