Avoid bad breakpoints with --gc-sections

We found a case where --gc-sections can cause gdb to set an invalid
breakpoint.  In the included test case, gdb will set a breakpoint with
two locations, one of which is 0x0.

The code in lnp_state_machine::check_line_address is intended to
filter out this sort of problem, but in this case, the entire CU is
empty, causing unrelocated_lowpc==0x0 -- which circumvents the check.

It seems to me that if a CU is empty like this, then it is ok to
simply ignore the line table, as there won't be any locations anyway.