gdb: Fix -Wuninitialized issue

gdb: Fix -Wuninitialized issue

I see the following warning when building GDB on FreeBSD/amd64 with
Clang 14,

======================================================================
  CXX    mdebugread.o
mdebugread.c:1069:3: error: variable 'f' is uninitialized when used here [-Werror,-Wuninitialized]
                f->set_loc_enumval (tsym.value);
                ^
mdebugread.c:836:17: note: initialize the variable 'f' to silence this warning
        struct field *f;
                       ^
                        = nullptr
======================================================================

after digging a little, I realized that we can not simply do what
Clang 14 says.

The root cause of this issue is that we lost the initialization of
the variable 'f' in this commit,

  commit 2774f2dad5f05e68771c07df6ab0fb23baa2118e
  Date:   Thu Aug 31 09:37:44 2023 +0200

      [gdb/symtab] Factor out type::{alloc_fields,copy_fields}

we have made these modifications,

 ---------------------------------------------------------------------
 --- a/gdb/mdebugread.c
 +++ b/gdb/mdebugread.c
 @@ -1034,9 +1034,7 @@ parse_symbol (SYMR *sh, union aux_ext *ax, char *ext_sh, int bigend,

         t->set_code (type_code);
         t->set_length (sh->value);
 -       t->set_num_fields (nfields);
 -       f = ((struct field *) TYPE_ALLOC (t, nfields * sizeof (struct field)));
 -       t->set_fields (f);
 +       t->alloc_fields (nfields, false);
 ---------------------------------------------------------------------

The problem is that the variable 'f' is used in the second half of
parse_symbol, that's why Clang complained.

To fix this issue we need to ensure that the varibale 'f' is
initialized.  Calling the fields method is an obvious way to fix this
issue.

Tested on FreeBSD/amd64 by rebuilding.

Approved-By: Tom de Vries <tdevries@suse.de>