Avoid undefined behavior regarding signed integers and left shifts.

Avoid undefined behavior regarding signed integers and left shifts.

* libguile/numbers.c (scm_logbit_p): If the requested bit is the sign
  bit (or above), check the sign portably.  Otherwise, ensure that we're
  testing the bit in a two's complement representation.
  (left_shift_exact_integer): Avoid left-shifting negative integers.

* libguile/vm-i-scheme.c (ash): Avoid left-shifting negative integers.