Fix substr_replace with slots in repl_ht being UNDEF

The check that was supposed to check whether the array slot was UNDEF
was wrong and never triggered. This resulted in a replacement w

Fix substr_replace with slots in repl_ht being UNDEF

The check that was supposed to check whether the array slot was UNDEF
was wrong and never triggered. This resulted in a replacement with the
empty string or the wrong string instead of the correct one. The correct
check pattern can be observed higher up in the function's code.

Closes GH-10323

Signed-off-by: George Peter Banyard <girgias@php.net>

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