buffer overflow in fetch_indexed_addr

oss-fuzz found another case where sanity checks weren't good enough.

	* dwarf.c (fetch_indexed_addr): Avoid integer overflow when
	sanity checking field offset
	(fetch_indexed_offset): Similarly.  Don't bother with "too small"
	warning, which will be covered by field offset check.  Warn
	about base and index rather than possibly overflowed calculation.