AsText() needs to know the maximum number of
characters a IEEE double precision value can
occupy to make sure there's enough buffer space.
The number was too small to hold all possible
values and this caused buffer overruns.
Fixed by correcting the calculation of the 
maximum digits in a string representation of an
IEEE double precision value as printed by 
String::qs_append(double).


mysql-test/r/gis.result:
  Bug #29166: test case
mysql-test/t/gis.test:
  Bug #29166: test case
sql/spatial.cc:
  Bug #29166: correct calculation of the maximum digits in
  a string representation of a double