ipfw: fix integer overflow in dummynet

[petrvaganoff]

Found during fuzzing using AFL++:

"strtoul()" returns a large positive number or a negative number.
Next, the sanitizer detects an integer overflow in
line 555 ("bw*= 1000") or in line 558 ("bw*= 1000000") or
in line 563 ("bw*= 8"), and the program crashes with
the error "SIGILL: illegal instruction operand".