dtc: Cleanup \nnn and \xNN string escape handling

Several small cleanups to the handling of octal and hex string
escapes:
	- Use strncmp() instead dof what were essentially open-coded
          versions of the same, with short fixed lengths.
	- The call path to get_oct_char() means an empty escape is not
          possible.  So replace the error message in this case with an
          assert.
	- Use die() instead of a non-fatal error message if
          get_hex_char() is given an empty escape.  Change error
          message to close match gcc's in the same circumstance.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>