Units_divide(u, u) and Units_power(u, 0) do not reduce to dimensionless

[cpiker]

Dividing a unit by itself, or raising one to the zeroth power, builds a unit string with
zero exponents instead of returning UNIT_DIMENSIONLESS, furthermore the string it
returns can't be parsed.

das_units a = Units_fromStr("V/m");
das_units d = Units_divide(a, a);   /* "V**0 m**0", not UNIT_DIMENSIONLESS */
das_units p = Units_power(a, 0);    /* same */

Both calls report:

ERROR: Error parsing units string 'V**0 m**0' at byte number 4
       (reported from das3/units.c:662, _Units_strToComponents)

and then return a usable-looking string pointer anyway. Units_reduce() on the
result logs a second error and returns the same string.

Expected results:

1. zero-exponent components should drop out of the result.
2. das2C shouldn't return strings it can't parse.

Where to look: das3/units.c, Units_divide() down into _Units_strToComponents()
near line 662, also check Units_power()