Introduce null_block_symbol

Introduce null_block_symbol

... in the spirit of null_ptid, null_frame_id, etc.

Fixes two instances of:

  /root/binutils-gdb/gdb/cp-namespace.c: In function 'block_symbol cp_lookup_nested_symbol(type*, const char*, const block*, domain_enum)':
  /root/binutils-gdb/gdb/cp-namespace.c:1010: warning: jump to case label
  /root/binutils-gdb/gdb/cp-namespace.c:1008: error:   crosses initialization of 'block_symbol <anonymous>'

Compiler info:

  Reading specs from /usr/lib/gcc-lib/amd64-unknown-openbsd5.8/4.2.1/specs
  Target: amd64-unknown-openbsd5.8
  Configured with: OpenBSD/amd64 system compiler
  Thread model: posix
  gcc version 4.2.1 20070719

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* cp-namespace.c (cp_lookup_bare_symbol)
	(cp_search_static_and_baseclasses, cp_lookup_symbol_via_imports)
	(cp_lookup_symbol_via_all_imports, cp_lookup_nested_symbol_1)
	(cp_lookup_nested_symbol): Use null_block_symbol.
	* d-namespace.c (d_lookup_symbol, d_lookup_nested_symbol)
	(d_lookup_symbol_imports, d_lookup_symbol_module): Use
	null_block_symbol.
	* symtab.c (null_block_symbol): New global.
	* symtab.h (null_block_symbol): Declare.