[bugfix] Fix crash in basic::subs().

[bugfix] Fix crash in basic::subs().

A regression in 1.7.0 was introduced with 1b8bcb06 in function
basic::subs_one_level(): implicitly casting *this to an ex first for
finding *this in m and later in the function's return statement caused
a crash in the second cast because *this was deleted in the first one.
After all, *this was dynamically allocated in basic::subs().

This bug was reported and hunted down by Mario Prausa.