Initialise all tpfa_solution members to zero (i.e., NULL).

This is in anticipation of introducing additional structure members to
represent solution variables associated with wells whilst having the
compiler generate the appropriate null pointers to aid debugging.

Unfortunately, GCC warns about the standard "structure = { 0 }" idiom
under -Wmissing-field-initializers which is enabled by default under
"-Wextra".

opm/core/pressure/IncompTpfa.cpp

@@ -118,7 +118,7 @@ namespace Opm
 	pressure.resize(grid_.number_of_cells);
 	faceflux.resize(grid_.number_of_faces);
 
-        ifs_tpfa_solution soln;
+        ifs_tpfa_solution soln = { 0 };
         soln.cell_press = &pressure[0];
         soln.face_flux  = &faceflux[0];