nm: avoid crash in print_lineno if func->name is NULL

This can occur when DW_AT_specification is used to refer to another DIE
that provides the actual DW_AT_name string. For example:

< 3><0x00000086> DW_TAG_subprogram

DW_AT_name              PrettyStackTraceEntry

...
< 1><0x00002cf4> DW_TAG_subprogram

DW_AT_specification     <0x00000086>

We will need to add support for DW_AT_specification, but in the interim
we should not segfault.

Obtained from: Elftoolchain (r3170)
Sponsored by: The FreeBSD Foundation