lttng.ust: Properly null-check the debug info state system

lttng.ust: Properly null-check the debug info state system

When opening a trace, some views (callstack etc.) might request
symbol/function names before the state system is properly loaded,
so the checkNotNull() check can fail. Actually reply that the
information is not available, instead of breaking the whole trace.

Change-Id: I9cb5446b00444d2b8388e9f21a5822187fd30734
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/76058
Reviewed-by: Hudson CI