Fix gdb_bfd_section_index for special sections

Fix gdb_bfd_section_index for special sections

Indexes returned for special sections are off by one, i.e. with N+4
sections last one has index N+4 returned which is outside allocated
obstack (at the same time index N is not used at all).

In worst case, if sections obstack is allocated up to end of chunk,
writing last section data will cause buffer overrun and some data
corruption.

Here's output from Valgrind::

==14630== Invalid write of size 8
==14630==    at 0x551B1A: add_to_objfile_sections_full (objfiles.c:225)
==14630==    by 0x552768: allocate_objfile (objfiles.c:324)
==14630==    by 0x4E8E2E: symbol_file_add_with_addrs (symfile.c:1171)
==14630==    by 0x4E9453: symbol_file_add_from_bfd (symfile.c:1280)
==14630==    by 0x4E9453: symbol_file_add (symfile.c:1295)
==14630==    by 0x4E94B7: symbol_file_add_main_1 (symfile.c:1320)
==14630==    by 0x514246: catch_command_errors_const (main.c:398)
==14630==    by 0x5150AA: captured_main (main.c:1061)
==14630==    by 0x51123C: catch_errors (exceptions.c:240)
==14630==    by 0x51569A: gdb_main (main.c:1164)
==14630==    by 0x408824: main (gdb.c:32)
==14630==  Address 0x635f3b8 is 8 bytes after a block of size 4,064 alloc'd
==14630==    at 0x4C2ABA0: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14630==    by 0x60F797: xmalloc (common-utils.c:41)
==14630==    by 0x5E787FB: _obstack_begin (obstack.c:184)
==14630==    by 0x552679: allocate_objfile (objfiles.c:294)
==14630==    by 0x4E8E2E: symbol_file_add_with_addrs (symfile.c:1171)
==14630==    by 0x4E9453: symbol_file_add_from_bfd (symfile.c:1280)
==14630==    by 0x4E9453: symbol_file_add (symfile.c:1295)
==14630==    by 0x4E94B7: symbol_file_add_main_1 (symfile.c:1320)
==14630==    by 0x514246: catch_command_errors_const (main.c:398)
==14630==    by 0x5150AA: captured_main (main.c:1061)
==14630==    by 0x51123C: catch_errors (exceptions.c:240)
==14630==    by 0x51569A: gdb_main (main.c:1164)
==14630==    by 0x408824: main (gdb.c:32)

gdb/ChangeLog:
	* gdb_bfd.c (gdb_bfd_section_index): Fix off-by-one for special
	sections.