Fix breakage in GEODEBUG debug code.
LINE doesn't have an "m" field (anymore anyway). Also fix unportable
assumption that %x can print the result of pointer subtraction.
In passing, improve single_decode() in minor ways:
* Remove unnecessary leading-whitespace skip (strtod does that already).
* Make GEODEBUG message more intelligible.
* Remove entirely-useless test to see if strtod returned a silly pointer.
* Don't bother computing trailing-whitespace skip unless caller wants
an ending pointer.
This has been broken since
261c7d4b653bc3e44c31fd456d94f292caa50d8f.
Although it's only debug code, might as well fix the 9.4 branch too.