rt: Fix a corner-case in unwinding that leads to stack overflow

In some scenarios upcall_rust_stack_limit fails to record the stack
limit, leaving it 0, and allowing subsequent Rust code to run into
the red zone.
auto merge of #8217 : brson/rust/reset_stack_limit, r=pcwalton

In some scenarios upcall_rust_stack_limit fails to record the stack
limit, leaving it 0, and allowing subsequent Rust code to run into
the red zone.