Fix floating-point input from decimal string.

Fix floating-point input from decimal string.

A bug was introduced in 3480230e: The divide-and-conquer method multiplies
with a power of the base, but this power is one too much if there is a
decimal point. This may happen because digits_to_I(...) is also called
from read_float(...). As a result, the number containd spurious zeros
(in the base used for reading it).

Thanks to Thomas Luthe <tluthe@physik.uni-bielefeld.de>.