[perl #120426] atof() small value rounding errors

For something like 0.153e-305, which is small, but not quite the smallest
number (which is around 2.2e-308), adding extra digits to the fractional part
could cause unnecessary rounding to zero.

From the bug report:

    $ echo 0.1530e-305 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
    v=0
    $ echo 0.153e-305  | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
    v=1.53e-306

This was because 0.1234e-305 is calculated as

    1234 / (10^309)

and 10^309 becomes infinity. In these edge cases, repeatedly decrement
the exponent and divide the mantissa by 10 until the exponent becomes in
range; in this case we instead calculate

    123 / (10^308)

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