memory leak in jjMAP/polynomial map

[dima@…]

This is also reported as ​https://github.com/Singular/Singular/issues/1090

ring r;
map F=r,x+y+z3,y+z+x2z3,z+1+xyz;
poly f=(x+y+z+xz)^10;
matrix m=f;
matrix mm;
while (1) {mm=F(m);}

doubles its RAM in size every 30 sec or so. Probably has to do with fast_map_common_subexp(). Found during work on ​https://trac.sagemath.org/ticket/27261, just as (already fixed) ​https://github.com/Singular/Singular/issues/1089 was.