Applying a complex in-place plan via '*' to a real vector should throw

My colleague sent me this MWE - it is about defining a complex in-place plan.
This should mutate vectors when applied via '*' as documented.
When applying this to a vector of floats it should throw, but in fact it
returns a complex vector but does not mutate the the vector.

```julia
using FFTW

plan = plan_fft!(zeros(ComplexF64, 4))
vec1 = ComplexF64[1,1,1,1]
vec2 = [1,1,1,1]

# changes vec1 as expected
plan * vec1; println(vec1,"\n")

# sends error as expected
try
FFTW.mul!(vec2, plan, vec2)
catch e;
println(e,"\n")
end

# does not say anything and does not change vec2
plan * vec2; println(vec2,"\n")
```

PR follows.

Run the Julia MWE in the issue and compare the `plan * vec2` path with the `FFTW.mul!` call that already raises an error. Trace the implementation of `*` for plans; it is done when applying the complex in-place plan to the real vector throws instead of silently returning a complex vector without mutating the input.

Written by the indexing model from the issue text.