Change dwarf2_frame_state_reg_info::reg to be std::vector

Change dwarf2_frame_state_reg_info::reg to be std::vector

This changes dwarf2_frame_state_reg_info::reg to be a std::vector.
This avoids passing NULL to memcpy in the copy constructor when the
original object does not have any registers.

gdb/ChangeLog
2018-10-03  Tom Tromey  <tom@tromey.com>

	* dwarf2-frame.h (dwarf2_frame_state_reg_info)
	<~dwarf2_frame_state_reg_info>: Update.
	<dwarf2_frame_state_reg_info>: Update.
	<alloc_regs>: Add assertion.  Update.
	<reg>: Now a std::vector.
	<num_regs>: Remove.
	<swap>: Update.
	* dwarf2-frame.c (dwarf2_restore_rule, execute_cfa_program)
	(execute_cfa_program_test, dwarf2_frame_cache): Update.