tests: Don't printf a known NULL symname in backtrace-dwarf.c.

GCC9 on 32bit systems might warn about '%s' directive argument is null
for symname in backtrace-dwarf.c. Just check whether symname is NULL.

This is an identical fix for the same issue as found in backtrace.c,
but now in backtrace-dwarf.c

Signed-off-by: Mark Wielaard <mark@klomp.org>
2 files changed