libdw: check that DWARF strings are null-terminated

libdw: check that DWARF strings are null-terminated

It is expected from libdw to return strings that are null-terminated to
avoid overflowing ELF data.

* Add calculation of a safe prefix inside string sections, where any
  string will be null-terminated.

* Check if offset overflows the safe prefix in dwarf_formstring.

Signed-off-by: Aleksei Vetrov <vvvvvv@google.com>