[Patch] Use "<unknown>" instead of NULL when a symbol isn't found

[Tom Tromey]

>>>>> "Sterling" == Sterling Augustine <saugustine@google.com> writes:

Tom> I wonder whether some correction or workaround is required elsewhere.

Sterling> There are various parts of linepec.c that assume that this field is
Sterling> never null, either by assertion or by dereferencing it. Any other
Sterling> workaround (at least for the case I'm dealing with) would probably
Sterling> just print an error message.

Yeah, I meant more whether the symbol-reading code should try to
diagnose and avoid broken debuginfo here.

Tom