Fix #275: FP underflow in reader allows restart with 0
If the floating-point underflow trap is enabled, signal an underflow error when the number would underflow to a denormal. We still signal a reader-error if the number would not fit in a float.
If the floating-point underflow trap is enabled, signal an underflow error when the number would underflow to a denormal. We still signal a reader-error if the number would not fit in a float.