src/query.c: Correct assertion in free_interim

src/query.c: Correct assertion in free_interim

!something is always >=0.  We meant to check that the free_interim
does not free more than was allocated.

Also do the assertion before the manipulation.  That reduces the
probability that a compiler will "prove" that the assertion is not
needed, or that it will fire too late.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>