simple crash with subst

[gorzel]

The errorneous input, a constant as second argument for subst, gives in general the errormessage " ? ringvar/par expected".

However, for rings with parameters a crash is caused, if the second argument is 0.

> ring r0=0,x,dp;
> subst(x,1,x);
   ? ringvar/par expected
   ? error occurred in or before STDIN line 2: `subst(x,1,x);`
> subst(x,0,x);
   ? ringvar/par expected
   ? error occurred in or before STDIN line 3: `subst(x,0,x);`
> ring rt = (0,t),x,dp;
> subst(x,1,x);        
   ? ringvar/par expected
   ? error occurred in or before STDIN line 5: `subst(x,1,x);`
> subst(x,0,x);        
Singular : signal 11 (v: 3150):
current line:>>subst(x,0,x);<<
Segment fault/Bus error occurred at 0 because of 10246 (r:1349799056)
please inform the authors
trying to restart...

(Even short: subst(1,0,1); shows the same behaviour.)

[hannes]

NULL dereference in jjSUBST_Test, fixed.