lavfi/scale: allocate interlaced scalers only if needed.

Fix "Value 0.000000 for parameter 'srch' out of range"
error message when source or destination height is 1.

Note: since the av_opt_set_int() calls are not checked for
failure and the interlaced scalers are not actually used,
this error has no consequence apart from a frightening message
in the log.
