Proactive reliability fix for broken FPUs: The base conversion functions

use log10() to calculate the size of the output array. The current code
has been tested on x86/amd64 (and to a lesser extent on qemu-mips qemu-sparc)
and produces sufficiently large values for all inputs tested so far (coefficient
sizes of 10**18 - 1 are hard to test exhaustively).

The new code does not rely on the correctness of log10() and resizes
the output arrays if the allocated space is insufficient.