charset.c: fix -Wpointer-sign

charset.c: fix -Wpointer-sign

$ make WERROR_CFLAGS="-Wpointer-sign -Werror" charset.o 2>&1 1>/dev/null
../../src/gdb/charset.c: In function ‘wchar_iterate’:
../../src/gdb/charset.c:665:13: error: pointer targets in assignment differ in signedness [-Werror=pointer-sign]
../../src/gdb/charset.c:691:13: error: pointer targets in assignment differ in signedness [-Werror=pointer-sign]
../../src/gdb/charset.c:706:12: error: pointer targets in assignment differ in signedness [-Werror=pointer-sign]

The encoding conversion code works with gdb_byte arrays as the generic
buffers that hold strings of any encoding/width.  Changing the type of
this field to gdb_byte* removes the need for one cast, and makes
everything work with the same types.  That's good -- WRT to strings,
"char *" is (almost) consistently throughout GDB only used for
ascii-ish strings.

gdb/
2013-03-07  Pedro Alves  <palves@redhat.com>

	* charset.c (struct wchar_iterator) <input>: Change type to 'const
	gdb_byte *'.
	(make_wchar_iterator): Remove cast to char*.
	(wchar_iterate): Change type of local.