There was a bug in float32_to_s that incorrectly mapped a zero exponent
to 0x38.  This meant 0.0f != 0.  At the same time, fix a generic type
punning bug in helper_memory_to_s and helper_s_to_memory.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>