Fix some crashes in hphpd

Summary: There are plenty of cores showing what is clearly the result of
popping an element from an empty stack, so guard popActiveLineBreak().

getActiveLineBreak() already checks for size() == 0, and is usually followed up
by a call to setActiveLineBreak(), which does not. So fix that too.

Reviewed By: @edwinsmith

Differential Revision: D1533549