Fix completion related libstdc++ assert when using -D_GLIBCXX_DEBUG

This commit fixes a libstdc++ assertion failure encountered when
running gdb.base/completion.exp.  In order to see this problem,
GDB must be built with the follow CFLAGS and CXXFLAGS as part
of the configure line:

  CFLAGS='-D_GLIBCXX_DEBUG' CXXFLAGS='-D_GLIBCXX_DEBUG'

(Also, this problem was encountered using Fedora rawhide.  It might
not be reproducible in Fedora versions prior to Fedora 34.)

Using the gdb.base/completion.exp test program, the problem can be
observed as follows:

[kev@rawhide-1 gdb]$ ./gdb -q testsuite/outputs/gdb.base/completion/completion
Reading symbols from testsuite/outputs/gdb.base/completion/completion...
(gdb) start
Temporary breakpoint 1 at 0x401179: file ../../worktree-master/gdb/testsuite/gdb.base/break.c, line 43.
Starting program: testsuite/outputs/gdb.base/completion/completion

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffd718, envp=0x7fffffffd728) at ../../worktree-master/gdb/testsuite/gdb.base/break.c:43
43	    if (argc == 12345) {  /* an unlikely value < 2^16, in case uninited */ /* set breakpoint 6 here */
(gdb) p <TAB>/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)

(Note that I added "<TAB>" to make it clear where the tab key was
pressed.)

gdb/ChangeLog:

	* ada-lang.c (ada_fold_name): Check for non-empty string prior
	to accessing it.
	(ada_lookup_name_info): Likewise.