range base: add savesub function

src/include/ranges/range_base.h

@@ -106,6 +106,12 @@ namespace CNORXZ
 	 */
 	virtual RangePtr sub(SizeT num) const;
 
+	/** Access sub-ranges.
+	    Same as sub(), but performs domain and memory safety checks.
+	    @param num size type argument
+	 */
+	virtual RangePtr savesub(SizeT num) const;
+	
 	/** Access sub-ranges.
 	    In the case the range is multi-dimensional this function returns an
 	    array of all sub-ranges (empty is one-dimensional)

src/lib/ranges/range_base.cc

@@ -70,6 +70,15 @@ namespace CNORXZ
 	return nullptr;
     }
 
+    RangePtr RangeBase::savesub(SizeT num) const
+    {
+	CXZ_ASSERT(num < dim(), "sub-range position = " << num
+		   << " exceeds range dimension = " << dim());
+	RangePtr o = sub(num);
+	CXZ_ASSERT(o != nullptr, "no sub-range at position = " << num);
+	return o;
+    }
+    
     MArray<RangePtr> RangeBase::sub() const
     {
 	return MArray<RangePtr>();