when reading from files of integers (or subranges thereof)

when reading from files of integers (or subranges thereof)
use sign-extending INDirect through buffer pointer
so the rangecheck works (which it doesn't if zero-extending is done,
e.g. for 1 byte reads becoming 2 bytes on the stack).

SCCS-vsn: usr.bin/pascal/src/proc.c 2.2