Avoid possible pointer wrap

PTR supplied to these macros can be read from user input, END is an
end of buffer pointer.  It's safer to do arithmetic on END than on PTR.

	* dwarf.c (SAFE_BYTE_GET): Check bounds by subtracting amount from
	END rather than adding amount to PTR.
	(SAFE_SIGNED_BYTE_GET, SAFE_BYTE_GET64): Likewise.