debugger: correct step_over for unbalanced calls/returns

Number of 'call' hook invocations is always greater than
number of 'return' hook events. (i.e. ffi functions does use
`call` but there is no `return`. Same is for builtin functions
like `assert` or `error`).

We count number of stack frames available, and stop once
level is once again less or equal to the necessary one.
Math.huge is used to stop on each line (trace mode).

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