Add some sanity checking in ECOFF lookup_line

Add some sanity checking in ECOFF lookup_line

More anti-fuzzer bounds checking for the ECOFF support.  A lot of this
is in ancient code using "long" for counts and sizes, which is why the
patch uses "(long) ((unsigned long) x + 1) > 0" in a few places.  The
unsigned long cast is so that "x + 1" doesn't trigger ubsan warnings
about signed integer overflow.  It would be a good idea to replace
most of the longs used in binutils with size_t, but that's more than I
care to do for COFF/ECOFF.

	* ecofflink.c (mk_fdrtab): Sanity check string offsets.
	(lookup_line): Likewise, and symbol indices.