fix slocus bug for zero ideal (see Singular-track 526)

fix 'slocus' bug for zero ideal (see Singular-track 526, http://www.singular.uni-kl.de:8002/trac/ticket/526)

In addition ASSUME statements were added which check 
if a current ring is supported by 'slocus'. 
(qrings rings and rings with integer coefficients are not supported)