Fix potential double free in link.c (#456)

* Fix potential double free in link.c

* Fix unittest

* Add link_attach_succeeds

* Add double-free test

* Fix return value in unit test function

* Address CR comments

* Fix unit tests

src/link.c

@@ -404,9 +404,9 @@ static void link_frame_received(void* context, AMQP_VALUE performative, uint32_t
                     }
                 }
             }
-        }
 
-        flow_destroy(flow_handle);
+            flow_destroy(flow_handle);
+        }
     }
     else if (is_transfer_type_by_descriptor(descriptor))
     {

tests/CMakeLists.txt

@@ -13,6 +13,7 @@ add_subdirectory(cbs_ut)
 add_subdirectory(connection_ut)
 add_subdirectory(frame_codec_ut)
 add_subdirectory(header_detect_io_ut)
+add_subdirectory(link_ut)
 add_subdirectory(message_ut)
 add_subdirectory(sasl_anonymous_ut)
 add_subdirectory(sasl_frame_codec_ut)

tests/link_ut/CMakeLists.txt

@@ -0,0 +1,18 @@
+#Copyright (c) Microsoft. All rights reserved.
+#Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+set(theseTestsName link_ut)
+set(${theseTestsName}_test_files
+${theseTestsName}.c
+)
+
+set(${theseTestsName}_c_files
+../../src/link.c
+)
+
+set(${theseTestsName}_h_files
+)
+
+build_c_test_artifacts(${theseTestsName} ON "tests/uamqp_tests")
+
+compile_c_test_artifacts_as(${theseTestsName} C99)