gdb: fix -Wmaybe-uninitialized warning in value.c

gdb: fix -Wmaybe-uninitialized warning in value.c

Since commit 11470e70ea0d ("gdb: store internalvars in an std::map"), bulding
with -O2, with g++ 11.3.0 on Ubuntu 22.04, I see:

      CXX    value.o
    In constructor ‘internalvar::internalvar(internalvar&&)’,
        inlined from ‘constexpr std::pair<_T1, _T2>::pair(_U1&&, _U2&&) [with _U1 = const char*&; _U2 = internalvar; typename std::enable_if<(std::_PCC<true, _T1, _T2>::_MoveConstructiblePair<_U1, _U2>() && std::_PCC<true, _T1, _T2>::_ImplicitlyMoveConvertiblePair<_U1, _U2>()), bool>::type <anonymous> = true; _T1 = const char*; _T2 = internalvar]’ at /usr/include/c++/11/bits/stl_pair.h:353:35,
        inlined from ‘constexpr std::pair<typename std::__strip_reference_wrapper<typename std::decay<_Tp>::type>::__type, typename std::__strip_reference_wrapper<typename std::decay<_Tp2>::type>::__type> std::make_pair(_T1&&, _T2&&) [with _T1 = const char*&; _T2 = internalvar]’ at /usr/include/c++/11/bits/stl_pair.h:572:72,
        inlined from ‘internalvar* create_internalvar(const char*)’ at /home/smarchi/src/binutils-gdb/gdb/value.c:1933:52:
    /home/smarchi/src/binutils-gdb/gdb/value.c:1831:8: warning: ‘<unnamed>.internalvar::u’ may be used uninitialized [-Wmaybe-uninitialized]
     1831 | struct internalvar
          |        ^~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/value.c: In function ‘internalvar* create_internalvar(const char*)’:
    /home/smarchi/src/binutils-gdb/gdb/value.c:1933:76: note: ‘<anonymous>’ declared here
     1933 |   auto pair = internalvars.emplace (std::make_pair (name, internalvar (name)));
          |                                                                            ^

This is because the union field internalvar::u is not initialized when
constructing the temporary internalvar object above.  That object is then used
for move-construction, and the (implicit) move constructor copies the
uninitialized bytes of field u over from the temporary object to the new
internalvar object.  The compiler therefore complains that we use uninitialized
bytes.  I don't think it's really a problem, because the internalvar object is
in the `kind == INTERNALVAR_VOID` state, in which the contents of the union is
irrelevant.  Still, mute the warning by default-initializing the union.

Change-Id: I70c392842f35255f50d8e63f4099cb6685366fb7
Reviewed-By: Tom Tromey <tom@tromey.com>