unity_fixture: Make unity_free() NULL-safe

At the start of unity_free(), check mem for NULL, and return immediately
if it is, so we don't crash in this case. This mimics the behaviour of
most free() implementations. Closes #135.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

extras/fixture/test/unity_fixture_Test.c

@@ -132,6 +132,11 @@ TEST(UnityFixture, PointerSet)
     TEST_ASSERT_POINTERS_EQUAL(&c2, p2);
 }
 
+TEST(UnityFixture, FreeNULLSafety)
+{
+  unity_free(NULL);
+}
+
 //------------------------------------------------------------
 
 TEST_GROUP(UnityCommandOptions);