Fix off-by-one error in ada_fold_name

I'm seeing a libstdc++ assertion failure when running GDB's "maint selftest"
command when GDB is configured with the following CFLAGS and CXXFLAGS as
part of the configure line:

  CFLAGS='-D_GLIBCXX_DEBUG -g3 -O0' CXXFLAGS='-D_GLIBCXX_DEBUG -g3 -O0'

This is what I see when running the self tests:

(gdb) maint selftest
Running selftest aarch64-analyze-prologue.
Running selftest aarch64-process-record.
Running selftest arm-record.
Running selftest arm_analyze_prologue.
Running selftest array_view.
Running selftest child_path.
Running selftest cli_utils.
Running selftest command_structure_invariants.
Running selftest copy_bitwise.
Running selftest copy_integer_to_size.
Running selftest cp_remove_params.
Running selftest cp_symbol_name_matches.
Running selftest dw2_expand_symtabs_matching.
/usr/include/c++/11/string_view:211: constexpr const value_type& std::basic_string_view<_CharT, _Traits>::operator[](std::basic_string_view<_CharT, _Traits>::size_type) const [with _CharT = char; _Traits = std::char_traits<char>; std::basic_string_view<_CharT, _Traits>::const_reference = const char&; std::basic_string_view<_CharT, _Traits>::size_type = long unsigned int]: Assertion '__pos < this->_M_len' failed.
Aborted (core dumped)

Here's a partial stack trace:

  #0  0x00007ffff6ef6262 in raise () from /lib64/libc.so.6
  #1  0x00007ffff6edf8a4 in abort () from /lib64/libc.so.6
  #2  0x00000000004249bf in std::__replacement_assert (
      __file=0xef7480 "/usr/include/c++/11/string_view", __line=211,
      __function=0xef7328 "constexpr const value_type& std::basic_string_view<_CharT, _Traits>::operator[](std::basic_string_view<_CharT, _Traits>::size_type) const [with _CharT = char; _Traits = std::char_traits<char>; std::ba"...,
      __condition=0xef7311 "__pos < this->_M_len")
      at /usr/include/c++/11/x86_64-redhat-linux/bits/c++config.h:2624
  #3  0x0000000000451737 in std::basic_string_view<char, std::char_traits<char> >::operator[] (this=0x7fffffffc200, __pos=8)
      at /usr/include/c++/11/string_view:211
  #4  0x00000000004329f5 in ada_fold_name (name="function")
      at /ironwood1/sourceware-git/rawhide-master/bld/../../worktree-master/gdb/ada-lang.c:988

And, looking at frame #4...

(top-gdb) up 4
    at /ironwood1/sourceware-git/rawhide-master/bld/../../worktree-master/gdb/ada-lang.c:988
988		fold_buffer[i] = tolower (name[i]);
(top-gdb) p i
$1 = 8
(top-gdb) p name.size()
$2 = 8

My patch adjusts the comparison to only copy name.size() characters
from the string.  I've added a separate statement for NUL character
termination of fold_buffer[].

gdb/ChangeLog:

	* ada-lang.c (ada_fold_name): Fix off-by-one error.

gdb/ada-lang.c

@@ -984,8 +984,9 @@ ada_fold_name (gdb::string_view name)
     {
       int i;
 
-      for (i = 0; i <= len; i += 1)
+      for (i = 0; i < len; i += 1)
 	fold_buffer[i] = tolower (name[i]);
+      fold_buffer[i] = '\0';
     }
 
   return fold_buffer;