With 21.2.1 I get:
(log #c(0 1) #c(0 -1))
Debugger received error of type: DIVISION-BY-ZERO #<a DIVISION-BY-ZERO 0x7f8925037640> Error flushed.
gcl, sbcl and ccl all return #C(-1.0 0.0).
other non-lisp software agrees.
i haven't yet tried master.
-JimC