_mul_overflow and get_encoded_value

A sufficiently mad compiler optimiser can take undefined behaviour
according to the C standard as an opportunity to remove code.  Since
"data + size" might be seen to be past the end of an array,
calculating such an expression is UB.

_mul_overflow is infrastructure for later patches.

	* bucomm.h (_mul_overflow): Define.
	* dwarf.c (get_encoded_value): Avoid pointer UB.