assert that array sizes are compatible

otherwise, when `A` and `b` were not compatible, I got an error that
didn’t seem informative.

ERROR: `canonicalspace` has no method matching
canonicalspace(::VectorDomainSpace{2,UltrasphericalSpace{0},Float64})
 in spaceconversion at
/Users/goretkin/.julia/v0.3/ApproxFun/src/Fun/FunctionSpace.jl:160
 in coefficients at
/Users/goretkin/.julia/v0.3/ApproxFun/src/Fun/Fun.jl:19
 in linsolve at
/Users/goretkin/.julia/v0.3/ApproxFun/src/Operators/linsolve.jl:92
 in linsolve at
/Users/goretkin/.julia/v0.3/ApproxFun/src/Operators/linsolve.jl:117