[gdb/exp] Fix assert when adding ptr to imaginary unit

[gdb/exp] Fix assert when adding ptr to imaginary unit

I'm running into this assertion failure:
...
$ gdb -batch -ex "p (void *)0 - 5i"
gdbtypes.c:3430: internal-error: \
  type* init_complex_type(const char*,   type*): Assertion \
  `target_type->code () == TYPE_CODE_INT \
   || target_type->code () == TYPE_CODE_FLT' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
...

This is a regression since commit c34e8714662 "Implement complex arithmetic".
Before that commit we had:
...
(gdb) p (void *)0 - 5i
Argument to arithmetic operation not a number or boolean.
...

Fix this in complex_binop by throwing an error, such that we have:
...
(gdb) print (void *)0 - 5i
Argument to complex arithmetic operation not supported.
...

Tested on x86_64-linux.

gdb/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	PR exp/27265
	* valarith.c (complex_binop): Throw an error if complex type can't
	be created.

gdb/testsuite/ChangeLog:

2021-02-05  Tom de Vries  <tdevries@suse.de>

	PR exp/27265
	* gdb.base/complex-parts.exp: Add tests.