Geo/Math.cpp: Fix potential uninitialized variable usage when using WGS84

  CXX     output/ANDROID7/src/Geo/Math.o
src/Geo/Math.cpp: In function 'GeoPoint FindLatitudeLongitude(const GeoPoint&, Angle, fixed)':
src/Geo/Math.cpp:495:31: warning: 'cos_2_sigma_m' may be used uninitialized in this function [-Wmaybe-uninitialized]
   fixed sin_sigma, cos_sigma, cos_2_sigma_m;
                               ^
src/Geo/Math.cpp:511:35: warning: 'cos_sigma' may be used uninitialized in this function [-Wmaybe-uninitialized]
   const fixed lat2 = atan2(sin_u1 * cos_sigma + cos_u1 * sin_sigma * cos_alpha1,
                                   ^
src/Geo/Math.cpp:511:68: warning: 'sin_sigma' may be used uninitialized in this function [-Wmaybe-uninitialized]
   const fixed lat2 = atan2(sin_u1 * cos_sigma + cos_u1 * sin_sigma * cos_alpha1,
                                                                    ^