gdb: fix crash when reading ECOFF debug information

gdb: fix crash when reading ECOFF debug information

In commit:

  commit 633cf2548bcd3dafe297e21a1dd3574240280d48
  Date:   Wed May 9 15:42:28 2018 -0600

      Remove cleanups from mdebugread.c

the following change was made in the function parse_partial_symbols in
mdebugread.c:

  -  fdr_to_pst = XCNEWVEC (struct pst_map, hdr->ifdMax + 1);
  -  old_chain = make_cleanup (xfree, fdr_to_pst);
  +  gdb::def_vector<struct pst_map> fdr_to_pst_holder (hdr->ifdMax + 1);
  +  fdr_to_pst = fdr_to_pst_holder.data ();

The problem with this change is that XCNEWVEC calls xcalloc, which in
turn calls calloc, and calloc zero initializes the allocated memory.
In contrast, the new line gdb::def_vector<struct pst_map> specifically
does not initialize the underlying memory.

This is a problem because, later on in this same function, we
increment the n_globals field within 'struct pst_map' objects stored
in the vector.  The incrementing is now being done from an
uninitialized starting point.

In this commit we switch from using gdb::def_vector to using
std::vector, this alone should be enough to ensure that the fields are
initialized to zero.

However, for extra clarity, I have also added initial values in the
'struct pst_map' to make it crystal clear how the struct will start
up.

This issue was reported on the mailing list here:

  https://sourceware.org/pipermail/gdb-patches/2021-November/183693.html

Co-Authored-By: Lightning <lightningth@gmail.com>