AirspaceParser, AngleDataField: omit the optional ParseDouble() parameter

nullptr is the default value.
Util/NumberParser: work around mingw64 wcstod() bug

mingw64 version 3.1 crashes when the "endptr" argument to wcstod() is
nullptr.  This patch adds another wrapper function which passes a
dummy parameter instead of nullptr.  This fixes instant crashes of our
6.7.4 PC binary (TRAC #3284).