#2197 mixed CRE / float arithmetic with keepfloat

[Barton Willis]

(%i4) rat(1.2) * rat(1/2) - 11/5, keepfloat : true;
(%o4)/R/ -8.0/5

(%i5) expand(%,0,0);
Maxima encountered a Lisp error:
Error in MACSYMA-TOP-LEVEL [or a callee]: -8.0 is not of type INTEGER.
Automatically continuing.
To enable the Lisp debugger set *debugger-hook* to nil.