gdb/dwarf: fix signature_type created with nullptr section

gdb/dwarf: fix signature_type created with nullptr section

Commit c44ab627b02 ("gdb/dwarf: pass section to dwarf2_per_cu_data
constructor") introduced a regression when using dwp.  It can be
reproduced with:

    $ make check TESTS="gdb.base/ptype-offsets.exp" RUNTESTFLAGS="--target_board=fission-dwp"

Then, to investigate:

    $ ./gdb  -nx -q --data-directory=data-directory testsuite/outputs/gdb.base/ptype-offsets/ptype-offsets -ex 'ptype int'
    Reading symbols from testsuite/outputs/gdb.base/ptype-offsets/ptype-offsets...
    /home/smarchi/src/binutils-gdb/gdb/dwarf2/read.c:3195:38: runtime error: member call on null pointer of type 'struct dwarf2_section_info'

Commit c44ab627b02 removed the assignment of signatured_type::section
(dwarf2_per_cu_data::section, really) in
fill_in_sig_entry_from_dwo_entry with the justification that the section
was already set when constructing the signatured_type.  Well, that was
true except for one spot in lookup_dwp_signatured_type which passes a
nullptr section to add_type_unit.

Fix that by passing the section to add_type_unit in that one spot.  This
is the same section that would have been set by
fill_in_sig_entry_from_dwo_entry before.

Add some asserts in the dwarf2_per_cu_data constructor to verity that
the passed dwarf2_per_bfd and dwarf2_section_info are non-nullptr.

Change-Id: If27dae6b4727957c96defc058c7e4be31472005b
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32739
Co-Authored-By: Tom de Vries <tdevries@suse.de>
Approved-By: Tom Tromey <tom@tromey.com>