buffer test: fix memory leak in test code

Forgot to delete a buffer in the test driver. There was no memory leak
or memory error in any installed code.

TODO

@@ -1,8 +1,6 @@
 Soon
 ====
 
- * libtests buffer test fails with address sanitizer.
-
  * Figure out how to render Gajić correctly in the PDF version of the
    qpdf manual.
 

libtests/buffer.cc

@@ -67,6 +67,7 @@ int main()
         Pl_Buffer bp3("bp3");
         b = bp3.getBuffer();
         std::cout << "size: " << b->getSize() << std::endl;
+        delete b;
     }
     catch (std::exception& e)
     {