#14901: Guard against crashes from corrupt Eclipse files and incomplete frustum

- RifEclipseOutputFileTools::timeSteps: avoid integer division by zero when
  the restart file has no INTEHEAD
- libecl ecl_grid: return NULL when the main EGRID grid cannot be created,
  and skip NNCHEAD blocks that cannot be read
- libecl well_info: skip wells when the restart header cannot be read
- cvf::Frustum::isOutside: do not assert on an incomplete frustum, cull nothing

Fixes #14901.

ApplicationLibCode/FileInterface/RifEclipseOutputFileTools.cpp

@@ -280,7 +280,7 @@ void RifEclipseOutputFileTools::timeSteps( const ecl_file_type*    ecl_file,
         }
     }
 
-    if ( perTimeStepHeaderKeywordCount )
+    if ( perTimeStepHeaderKeywordCount && !timeSteps->empty() )
     {
         // 6X simulator can report more then one keyword block per time step. Report the total number of keywords per
         // time steps to be able to read data from correct index