libstdc++: Do not call log10(0.0) in std::format [PR110860]

Calling log10(0.0) returns -inf which has undefined behaviour when
converted to an integer. We only need to use log10 for large values
anyway. If the value is zero then the larger buffer is only needed due
to a large precision, so we don't need to use log10 to estimate the
number of digits for the significand.

libstdc++-v3/ChangeLog:

	PR libstdc++/110860
	* include/std/format (__formatter_fp::format): Do not call log10
	with zero values.