runtime: skip functions with no lines when building src line table

runtime: skip functions with no lines when building src line table

Avoid getting out of synch when a function, such as main.init,
has no associated line number information.  Without this the
function before main.init can skip the PC all the way to the
next function, which will cause the next function's line table
to be associated with main.init, and leave subsequent
functions with the wrong line numbers.

R=rsc
CC=golang-dev
https://golang.org/cl/4426055