Avoid transforming uninitalized data.

In r2c/c2r transforms when using 4-way SIMD, sometimes FFTW uses the
following hack: to transform an odd number of inputs, it copies the
input into a buffer that holds space for one extra input; it
transforms the buffer (now comprising an even number of inputs, as
required by SIMD); it copies back the odd number of transformed
inputs, ignoring the padding element.

The extra input was uninitialized until now.  This is ok because we
ignore the transform of the uninitialized input.  Transforming
uninitialized data may cause floating-point exceptions, an effect that
is observable.  This patch initializes the additional elements to
zero, thus avoiding the problem.

This patch also includes a test, but the test is disabled by default
because it is nonportable.  To observe the FP exception, one must use
feenableexcept(), which appears to be a GNU-ism.