DenseStorage safely copy/swap.

Fixes #2229.

For dynamic matrices with fixed-sized storage, only copy/swap
elements that have been set.  Otherwise, this leads to inefficient
copying, and potential UB for non-initialized elements.

test/SafeScalar.h

@@ -0,0 +1,30 @@
+
+// A Scalar that asserts for uninitialized access.
+template<typename T>
+class SafeScalar {
+ public:
+  SafeScalar() : initialized_(false) {}
+  SafeScalar(const SafeScalar& other) {
+    *this = other;
+  }
+  SafeScalar& operator=(const SafeScalar& other) {
+    val_ = T(other);
+    initialized_ = true;
+    return *this;
+  }
+  
+  SafeScalar(T val) : val_(val), initialized_(true) {}
+  SafeScalar& operator=(T val) {
+    val_ = val;
+    initialized_ = true;
+  }
+  
+  operator T() const {
+    VERIFY(initialized_ && "Uninitialized access.");
+    return val_;
+  }
+ 
+ private:
+  T val_;
+  bool initialized_;
+};