amd64: remove additional comparison for validity of a register number.

Second part of the && is already guaranteed in the "regnum < num_regs"
due to comparison above.

2017-01-27  Walfred Tedeschi  <walfred.tedeschi@intel.com>

	* amd64-nat.c (amd64_native_gregset_reg_offset): Simplify logic
	that checks validity of a register number.