Fix GH-14290: Member access within null pointer in extension spl

php_pcre_replace_impl() can fail and return NULL. We should take that
error condition into account. Because other failure

Fix GH-14290: Member access within null pointer in extension spl

php_pcre_replace_impl() can fail and return NULL. We should take that
error condition into account. Because other failures return false, we
return false here as well.

At first, I also thought there was a potential memory leak in the error
check of replacement_str, but found that the error condition can never
trigger, so replace that with an assertion.

Closes GH-14292.

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