Fix PR gdb/20948: --write option to GDB causes segmentation fault

Fix PR gdb/20948: --write option to GDB causes segmentation fault

When opening a BFD for update, as gdb --write does, modifications to
anything but the contents of sections is restricted.

Do not try to write back any ELF headers in this case.

bfd/ChangeLog
2018-09-24  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	PR gdb/20948
	* elf.c (_bfd_elf_write_object_contents): Return from function
	early if abfd->direction == both_direction.

gdb/testsuite/ChangeLog
2018-09-24  Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	PR gdb/20948
	* gdb.base/write_mem.exp: New test.
	* gdb.base/write_mem.c: Likewise.