agilent-dmm: Fix SEGV during incomplete reply from meter.

If buf contains exactly the string "Agilent Technologies" (for example if
there are bugs in the timeout logic or serial driver causing the reply from
the meter to be abridged) then this code will SEGV. This is because tokens[1]
is NULL but only tokens[2] and tokens[3] (both of which are undefined) are
NULL checked.

Can be trivially corrected by NULL checking tokens[1] as well.

Signed-off-by: Daniel Thompson <redacted>