Fix mixed disassembly showing source lines for "line 0"

"line 0" in a DWARF linetable means something that doesn't have associated
source. The code for mixed disassembly has a comment indicating that
"line 0" should be skipped, but the wrong value was returned. Fix the return
value and add a test to check that we don't incorrectly show source lines
from the beginning of the file.

Reviewed By: jasonmolenda

Differential Revision: https://reviews.llvm.org/D112931