[llvm-nm][MachO] Don't call getFlags on redacted symbols

Avoid calling getFlags on a non-existent symbol.

The way this is triggered is by calling strip -N on a binary, which sets
the MH_NLIST_OUTOFSYNC_WITH_DYLDINFO header flag. Then, in the
LC_FUNCTION_STARTS command, nm is trying to print the stripped symbols
and needs the proper checks.