#3116 solve gives error for certain systems of underdetermined equations

[bigfooted]

When you give solve a system of underdetermined equations containing undefined variables, it sometimes gives an error.

Example:

solve([a((-x-2)sqrt(b/a)+y),(x-2)b-ya*sqrt(b/a)],[x,y]);

expt: undefined: 0 to a negative exponent.
-- an error. To debug this try: debugmode(true);

Solve returns [] when I assume something for a or b, e.g. assume(a<1).
The command to_poly_solve returns %union() without an assume.

Extra information supplied by Robert Dodier:

Well, this is an underdetermined system of equations, which solve can
handle, if it notices. The bug here is that solve didn't see that and it
ends up trying to divide by something like the determinant of the
coefficient matrix.