php: OOB read in grapheme_stripos and grapheme_strpos when negative offset is used

The graphemestripos function in ext/intl/grapheme/graphemestring.c in PHP before 5.5.35, 5.6.x before 5.6.21, and 7.x before 7.0.6 allows remote attackers to cause a denial of service out-of-bounds read or possibly have unspecified other impact via a negative offset...