[dwarf] Anonymous nested function causes SEGV during psymbol read

[dwarf] Anonymous nested function causes SEGV during psymbol read

According to the DWARF3 standard, a function always has a name attribute
(Section 3.3 - Subroutine and Entry Point Entries).  The only exception
is when a DW_AT_abstract_origin attribute is provided, in which case
the name may be inherited from the referenced DIE.

The problem occured because our compiler generated a subprogram DIE
for a nested function where the name attribute was missing (and no
abstract-origin either).  Our code in add_partial_symbol is not
prepared to deal with the situation, and happily just tries  to compute
the length of the (NULL) function name.

This normally cannot happen, because there is already a guard in
scan_partial_symbols, where we (silently!) ignore anonymous dies,
including anonymous subprograms. Unfortunately, there is a flaw that
affects Ada and other languages that allow nested subprograms. For
nested subprograms, we do not go through scan_partial_symbols and
thus we are missing the name check.

This patch adds the name check in the nested subprogram case. It also
adds a complaint which is emitted during the psymtab->symtab conversion
phase.

gdb/ChangeLog:

        * dwarf2read.c (add_partial_subprogram): Make sure the subprogram
        DIE has a name before creating the associated partial symbol.
        (read_func_scope): Emit a complaint if the subprogram does not
        have a name or when we can't extract the subprogram PC bounds.

gdb/testsuite/ChangeLog:

        * gdb.dwarf2/dw2-anonymous-func.S: New file.
        * gdb.dwarf2/dw2-anonymous-func.exp: New testcase.

Tested on x86_64-linux, no regression.  Note that the testcase also
verifies that the psymtab->symtab conversion does not crash (this is
the purpose of the "list file1.txt:1" test.