gdb: fix "passing NULL to memcpy" UBsan error in dwarf2/cooked-index.c

gdb: fix "passing NULL to memcpy" UBsan error in dwarf2/cooked-index.c

Reading a simple file compiled with :

    $ gcc -DONE=1 -gdwarf-4 -g3  test.c
    $ gcc --version
    gcc (Ubuntu 9.4.0-1ubuntu1~20.04) 9.4.0

I get:

    Reading symbols from /tmp/cwd/a.out...
    /home/smarchi/src/binutils-gdb/gdb/dwarf2/cooked-index.c:332:11: runtime error: null pointer passed as argument 2, which is declared to never be null

It looks like even if the size is 0 (the size of the `entries` vector is
0), we shouldn't be passing a NULL pointer to memcpy.  And
`entries.data ()` returns NULL.

Fix that by using std::vector::insert to insert the items of entries
into m_entries.  I haven't checked, but it should essentially compile
down to a memcpy, since the vector elements are trivially copyiable.

Change-Id: I75f1c901e9b522e42e89eb5936e2c70d68eb21e5