1.0.6.13: minor fix to the compiler's interval-arithmetic

1.0.6.13: minor fix to the compiler's interval-arithmetic

 * When calculating the interval of (* some-number constant-zero),
   multiply the signums of the interval endpoints, not the numbers
   themselves: this avoids compile-time breakage when the zero is a
   float and the non-zero number is too large to be represented as a
   float.