Add null check for character type name

The internal AdaCore test suite found that
wchar_printer::get_default_encoding checks the name of a type without
first seeing if the name is null:

  else if (streq (chtype->name (), "wchar_t"))

Mysteriously, though, this only seems to happen for certain targets
and perhaps only under certain conditions (I couldn't reproduce it
msyelf).

I felt that the fix is fairly obvious and that this isn't really worth
writing a test for.  However I can write one using the DWARF assembler
if someone feels it is worthwhile.