Fix regression in raster psuedo ramp rendering when only two classes …

…exist

src/core/raster/qgscolorrampshader.cpp

@@ -359,9 +359,6 @@ bool QgsColorRampShader::shade( double value, int *returnRedValue, int *returnGr
     mLUTInitialized = true;
   }
 
-  if ( mLUT.empty() )
-    return false;
-
   // overflow indicates that value > maximum value + DOUBLE_DIFF_THRESHOLD
   // that way idx can point to the last valid item
   bool overflow = false;
@@ -380,6 +377,10 @@ bool QgsColorRampShader::shade( double value, int *returnRedValue, int *returnGr
       overflow = true;
     }
   }
+  else if ( lutIndex < 0 )
+  {
+    return false;
+  }
   else
   {
     // get initial value from LUT