[gdb/symtab] Fix segfault with dwp file

Consider the following test-case:
...
$ cat test.c
int main (void) { return 0; }
$ clang -g -gsplit-dwarf test.c -o test
$ llvm-dwp -e test -o test.dwp
...

This runs into a segmentation fault:
...
$ gdb -q -batch test
Fatal signal: Segmentation fault
...

The segmentation fault happens because in read_dwo_str_index this line sets p
to nullptr:
...
  const gdb_byte *p = reader->dwo_file->sections.str_offsets.buffer;
...
while the following code expects it to point to some data.

The section we're trying to read is:
...
(gdb) p reader->dwo_file->sections.str_offsets
$4 = {s = {section = 0xffffcc00a9d0, containing_section = 0xffffcc00a9d0},
  buffer = 0x0, size = 28, virtual_offset = 0, readin = false, is_virtual = true}
...

At first glance, the section is not readin, but actually it is.

This is a virtual section, meaning part of a containing section:
...
(gdb) p *reader->dwo_file->sections.str_offsets.s.containing_section
$8 = {s = {section = 0xffffcc00cde8, containing_section = 0xffffcc00cde8},
  buffer = 0xffffcc009650 "\030", size = 28, virtual_offset = 0, readin = true,
  is_virtual = false}
...
which is readin.

Fix this in create_dwp_v2_or_v5_section by initializing the buffer of the
virtual section using the buffer of the containing section:
...
  result.buffer = section->buffer + offset;
...

Unfortunately it's difficult to write a test-case for this.  We'll have to
teach the dwarf assembler to generate dwp files.

Tested on aarch64-linux.

This is a partial fix for PR symtab/31497.

Approved-By: Tom Tromey <tom@tromey.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31497