Fix segfault resulting from empty print prompt

Summary:
I have found a way to segfault lldb in 7 keystrokes! Steps to reproduce:
1) Launch lldb
2) Type `print` and hit enter. lldb will now prompt you to type a list of
    expressions, followed by an empty line.
3) Hit enter, indicating the end of your input.
4) Segfault!

After some investigation, I've found the issue in Host/common/Editline.cpp.
Editline::MoveCursor() relies on m_input_lines not being empty when the `to`
argument is CursorPosition::BlockEnd. This scenario, as far as I can tell,
occurs in one specific instance: In Editline::EndOrAddLineCommand() when the
list of lines being processed contains exactly one string (""). Meeting this
condition is fairly simple, I have posted steps to reproduce above.

Reviewers: krytarowski, zturner, labath

Reviewed By: labath

Subscribers: scott.smith, lldb-commits

Differential Revision: https://reviews.llvm.org/D32421
Patch by Alex Langford.

llvm-svn: 302225