Handle complex error type in read_base_type

Handle complex error type in read_base_type

It turns out there was one more bug in the earlier complex series:
read_base_type could cause an assertion failure on some platforms.  I
found this running the AdaCore internal test suite, but you can also
see it by running gdb's "gdb.cp" tests for x86 (not x86-64).

In particular, the DW_ATE_complex_float case calls
dwarf2_init_complex_target_type, which calls dwarf2_init_float_type,
which can return a type using TYPE_CODE_ERROR.

This patch changes the DWARF reader to handle this case, the same way
that the f-lang.c patch did.  Perhaps init_complex_type really should
be changed to allow TYPE_CODE_ERROR?  I was not sure.

Tested on x86-64 Fedora 30, using an x86 build.  I'm checking this in.

gdb/ChangeLog
2020-04-06  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (read_base_type) <DW_ATE_complex_float>: Handle
	TYPE_CODE_ERROR.