Fix segfault in mb_strrpos/mb_strripos with ASCII encoding and negative offset

We're setting the encoding from PHP_FUNCTION(mb_strpos), but mbfl_strpos would
discard it, setting it to mb

Fix segfault in mb_strrpos/mb_strripos with ASCII encoding and negative offset

We're setting the encoding from PHP_FUNCTION(mb_strpos), but mbfl_strpos would
discard it, setting it to mbfl_encoding_pass, making zend_memnrstr fail due to a
null-pointer exception.

Fixes GH-11217
Closes GH-11220

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