Tab completing inside the multiline expression command can cause LLDB to crash. The easiest way
to do this is to go inside a frame with at least one local variable and then try to complete:
(lldb) expr 1. a[tab]
Reason for this was some mixup when we calculate the cursor position. Obviously we should calculate
the offset inside the string by doing 'end - start', but we are doing 'start - end' (which causes the offset to
become -1 which will lead to some out-of-bounds reading).
I don't see any way to test this as the *multiline* expression completion is completely untested at the moment
and I don't think we have any existing code for testing infrastructure for it.