[sim,rx] Silence warning that turns into a build error

[sim,rx] Silence warning that turns into a build error

On a 32-bit build, I ran into the following:

sim/rx/fpu.c:789:6: error: "*((void *)&a+8)" may be used uninitialized in this function [-Werror=maybe-uninitialized]
rv = fp_implode (&a);

To silence this, just initialize the struct with 0's.

sim/rx/ChangeLog:

2021-04-09  Luis Machado  <luis.machado@linaro.org>

	* fpu.c (rxfp_itof): Initialize structure.