ext/mysqlnd: Use zval_try_get_string() instead of converting zval

[Girgias]

No description provided.

[kamil-tekiela]

It looks fine to me, although I am not sure what difference does it make here.

[nielsdos]

This is wrong because the string is never freed it will leak memory for non-interned strings.
Probably should do an in-place "try" convert instead of getting a separate string?
Note that row data is managed by mysqlnd so that will destroy the zval

[devnexen]

she knows.