[Raster] Do not crash when displaying a raster with complex datatype (fixes #16405)

That said, complex datatypes are not handled, so QGIS cannot do anything useful
with such datasets.

Author: rouault

src/core/raster/qgscontrastenhancement.cpp

@@ -180,7 +180,10 @@ int QgsContrastEnhancement::enhanceContrast( double theValue )
 
   if ( mLookupTable && NoEnhancement != mContrastEnhancementAlgorithm )
   {
-    return mLookupTable[static_cast <int>( theValue + mLookupTableOffset )];
+    double shiftedValue = theValue + mLookupTableOffset;
+    if ( shiftedValue >= 0 && shiftedValue < mRasterDataTypeRange + 1 )
+      return mLookupTable[static_cast <int>( shiftedValue )];
+    return 0;
   }
   else
   {

src/core/raster/qgscontrastenhancementfunction.cpp

@@ -49,12 +49,8 @@ int QgsContrastEnhancementFunction::enhance( double theValue )
 bool QgsContrastEnhancementFunction::isValueInDisplayableRange( double theValue )
 {
   //A default check is to see if the provided value is with the range for the data type
-  if ( theValue < QgsContrastEnhancement::minimumValuePossible( mQgsRasterDataType ) || theValue > QgsContrastEnhancement::maximumValuePossible( mQgsRasterDataType ) )
-  {
-    return false;
-  }
-
-  return true;
+  // Write the test as ( v >= min && v <= max ) so that v = NaN returns false
+  return theValue >= QgsContrastEnhancement::minimumValuePossible( mQgsRasterDataType ) && theValue <= QgsContrastEnhancement::maximumValuePossible( mQgsRasterDataType );
 }
 
 void QgsContrastEnhancementFunction::setMaximumValue( double theValue )