libdwfl: Return failure from dwfl_standard_find_debuginfo for NULL module.

libdwfl: Return failure from dwfl_standard_find_debuginfo for NULL module.

GCC10 -fanalyzer plus -flto notices that some functions called by
dwfl_standard_find_debuginfo check that the given module isn't NULL,
but others expect it to be non-NULL. Just return a failure immediately
when a NULL mod is passed to dwfl_standard_find_debuginfo.

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