Bug#12985021 SIMPLE QUERY WITH DECIMAL NUMBERS TAKE AN

When parsing the fractional part of a string which
is to be converted to double, we can stop after a few digits:
the extra digits will not contribute to the actual result anyways.


mysql-test/r/func_str.result:
  New tests.
mysql-test/t/func_str.test:
  New tests.
strings/dtoa.c:
  The problem was s2b() multiplying and adding hundreds-of-thousands
  of ever smaller fractions.