vector_base: fix memory leak in reserve()

Currently, when using reserve() to actually grow the
storage in a vector, a new backend storage is allocated,
but the old one is never deallocated. A more minor unexpected
behavior that also occurred is that the vector would still apply
its exponential growth behavior, ie., it when taking a vector of
size 3 and calling `.reserve(4)` on it, one would actually end up
with a vector of size 6.

The logic to allocate the new storage, copy in the existing elements,
then destroy the old elements and swap in the new storage (which does
to the old storage being destructed and hence deallocated) is, essentially,
taken from how `.resize()` works. When growing a vector using `.reserve()`,
the new capacity will now be exactly what was specified in the call to
`.reserve()`.

thrust/detail/vector_base.inl

@@ -364,7 +364,38 @@ template<typename T, typename Alloc>
 {
   if(n > capacity())
   {
-    allocate_and_copy(n, begin(), end(), m_storage);
+    // compute the new capacity after the allocation
+    size_type new_capacity = n;
+
+    // do not exceed maximum storage
+    new_capacity = thrust::min THRUST_PREVENT_MACRO_SUBSTITUTION <size_type>(new_capacity, max_size());
+
+    // create new storage
+    storage_type new_storage(copy_allocator_t(), m_storage, new_capacity);
+
+    // record how many constructors we invoke in the try block below
+    iterator new_end = new_storage.begin();
+
+    try
+    {
+      // construct copy all elements into the newly allocated storage
+      new_end = m_storage.uninitialized_copy(begin(), end(), new_storage.begin());
+    } // end try
+    catch(...)
+    {
+      // something went wrong, so destroy & deallocate the new storage
+      new_storage.destroy(new_storage.begin(), new_end);
+      new_storage.deallocate();
+
+      // rethrow
+      throw;
+    } // end catch
+
+    // call destructors on the elements in the old storage
+    m_storage.destroy(begin(), end());
+
+    // record the vector's new state
+    m_storage.swap(new_storage);
   } // end if
 } // end vector_base::reserve()