Backport fix for bug #70066

Given the number of duplicates this bug report had, it seems
worthwhile to fix this on PHP-7.4 as well.

Cherry-pick of 106e7e4bca7c0fd975eb219b18e3c3

Backport fix for bug #70066

Given the number of duplicates this bug report had, it seems
worthwhile to fix this on PHP-7.4 as well.

Cherry-pick of 106e7e4bca7c0fd975eb219b18e3c34957ba8657.

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Fixed bug #70066

If we fall back to emulated prepared statements, destroy S->stmt,
so the code doesn't get confused about which mode we're in.