Add support for checking band validity.

Author: abellgithub

gcore/gdal_priv.h

@@ -2068,6 +2068,11 @@ class CPL_DLL GDALRasterBand : public GDALMajorObject
 
     ~GDALRasterBand() override;
 
+    bool IsValid() const
+    {
+        return static_cast<bool>(poDS);
+    }
+
     int GetXSize() const;
     int GetYSize() const;
     int GetBand() const;