sigsegv protection (exp overflow)
don't return inf!
use errno=EOVERFLOW to signal an overflow (as my_strntod uses errno anyway)
if errno will be too slow, my_strtod can be changed to return overflow status in a parameter (like my_strntod does)
- get my_strtod to return inf
- get Field_float::store(double) and Field_double::store(float) to set null for
nan value
(as extra serg's recomendations to fix for patch on
Bug #2082 'mysqldump converts "inf" to null')
