Avoid crash in dwarf2_init_complex_target_type

After commit 35add35 ("gdb: Fix failure in gdb.base/complex-parts.exp
for x86-32"), dwarf2_init_complex_target_type can crash if "tt" is
nullptr.  This patch avoids the problem by checking for this case.

No test case because I don't know a good way to write one; it was
found by an internal AdaCore test case that apparently uses a 16 bit
floating point type.

gdb/ChangeLog:
	* dwarf2read.c (dwarf2_init_complex_target_type): Check "tt"
	against nullptr before use.

gdb/ChangeLog
2019-04-17  Tom Tromey  <tromey@adacore.com>

	* dwarf2read.c (dwarf2_init_complex_target_type): Check "tt"
	against nullptr before use.