lttng: Handle unknown line numbers in addr2line output

lttng: Handle unknown line numbers in addr2line output

addr2line can return "filename.c:?" when it knows a file name
but not its number. Do not attempt to parse "?" as a Long, you
will fail miserably.

Also change the odd/even flip to the start of the loop: the
presence of "continue" statements could skip that operation.

Change-Id: Ia685e91833c6c472decfd3fd21acf7ad70d1ae34
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/72118
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>