[gdb/symtab] Fix assert in write_one_signatured_type

[gdb/symtab] Fix assert in write_one_signatured_type

When running test-case gdb.dwarf2/fission-reread.exp with target board
cc-with-gdb-index, we run into an abort during the generation of the gdb-index
by cc-with-tweaks.sh:
...
build/gdb/testsuite/cache/gdb.sh: line 1: 27275 Aborted  (core dumped)
...

This can be reproduced on the command line like this:
...
$ gdb -batch ./outputs/gdb.dwarf2/fission-reread/fission-reread \
  -ex 'save gdb-index  ./outputs/gdb.dwarf2/fission-reread'
warning: Could not find DWO TU fission-reread.dwo(0x9022f1ceac7e8b19) \
  referenced by TU at offset 0x0 [in module fission-reread]
warning: Could not find DWO CU fission-reread.dwo(0x807060504030201) \
  referenced by CU at offset 0x561 [in module fission-reread]
Aborted (core dumped)
...

The abort is a segfault due to a using a nullptr psymtab in
write_one_signatured_type.

The problem is that we're trying to write index entries for the type unit
with signature:
...
(gdb) p /x entry->signature
$2 = 0x9022f1ceac7e8b19
...
which is a skeleton type unit:
...
Contents of the .debug_types section:

  Compilation Unit @ offset 0x0:
   Length:        0x4a (32-bit)
   Version:       4
   Abbrev Offset: 0x165
   Pointer Size:  4
   Signature:     0x9022f1ceac7e8b19
   Type Offset:   0x0
 <0><17>: Abbrev Number: 2 (DW_TAG_type_unit)
    <18>   DW_AT_comp_dir    : /tmp/src/gdb/testsuite
    <2f>   DW_AT_GNU_dwo_name: fission-reread.dwo
    <42>   DW_AT_GNU_pubnames: 0x0
    <46>   DW_AT_GNU_pubtypes: 0x0
    <4a>   DW_AT_GNU_addr_base: 0x0
...
referring to a .dwo file, but as the warnings show, the .dwo file is not
found.

Fix this by skipping the type unit in write_one_signatured_type if
psymtab == nullptr.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-02-02  Tom de Vries  <tdevries@suse.de>

	PR symtab/24620
	* dwarf2/index-write.c (write_one_signatured_type): Skip if
	psymtab == nullptr.

gdb/testsuite/ChangeLog:

2021-02-02  Tom de Vries  <tdevries@suse.de>

	PR symtab/24620
	* gdb.dwarf2/fission-reread.exp: Add test-case.