Fix -D_GLIBCXX_DEBUG gdb-add-index regression

Fix -D_GLIBCXX_DEBUG gdb-add-index regression

Fedora Rawhide started to use -D_GLIBCXX_DEBUG which made gdb-add-index
failing:
	gdb: Out-of-bounds vector access while running gdb-add-index
	https://bugzilla.redhat.com/show_bug.cgi?id=1540559

/usr/include/c++/7/debug/safe_iterator.h:270:
Error: attempt to dereference a past-the-end iterator.
Objects involved in the operation:
    iterator "this" @ 0x0x7fffffffcb90 {
      type = __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<unsigned char*, std::__cxx1998::vector<unsigned char, gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> > > >, std::__debug::vector<unsigned char, gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> > > > (mutable iterator);
      state = past-the-end;
      references sequence with type 'std::__debug::vector<unsigned char, gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> > >' @ 0x0x7fffffffcc50
    }

/usr/include/c++/7/debug/vector:417:
Error: attempt to subscript container with out-of-bounds index 556, but
container only holds 556 elements.
Objects involved in the operation:
    sequence "this" @ 0x0x2e87af8 {
      type = std::__debug::vector<partial_symbol*, std::allocator<partial_symbol*> >;
    }

The two -D_GLIBCXX_DEBUG regressions were made by:

commit bc8f2430e08cc2a520db49a42686e0529be4a3bc
Author: Jan Kratochvil <jan.kratochvil@redhat.com>
Date:   Mon Jun 12 16:29:53 2017 +0100
    Code cleanup: C++ify .gdb_index producer

commit af5bf4ada48ff65b6658be1fab8f9c8f8ab5f319
Author: Simon Marchi <simon.marchi@ericsson.com>
Date:   Sat Oct 14 08:06:29 2017 -0400
    Replace psymbol_allocation_list with std::vector

gdb/ChangeLog
2018-04-12  Jan Kratochvil  <jan.kratochvil@redhat.com>

	PR gdb/23053
	* dwarf-index-write.c (data_buf::grow) (write_one_signatured_type)
	(recursively_write_psymbols) (debug_names::recursively_write_psymbols)
	(debug_names::write_one_signatured_type): Fix -D_GLIBCXX_DEBUG
	regression.