CDRIVER-3008 fix possible leak in run_json_general_test (#1347)

* CDRIVER-3008 fix possible leak in test

* document example use of `MONGOC_JSON_SUBTEST`

Author: kevinAlbs

CONTRIBUTING.md

@@ -296,7 +296,12 @@ Specification tests may be filtered by their description:
 
 * `MONGOC_JSON_SUBTEST=<string>`
 
-This can be useful in debugging a specific test case in a spec test file with multiple tests.
+This can be useful in debugging a specific test case in a spec test file with multiple tests. Example:
+
+```sh
+MONGOC_JSON_SUBTEST="Insert with randomized encryption, then find it" \
+  ./cmake-build/src/libmongoc/test-libmongoc -l "/client_side_encryption/legacy/basic"
+```
 
 To test with a declared API version, you can pass the API version using an environment variable:
 

src/libmongoc/tests/json-test.c

@@ -1824,6 +1824,7 @@ run_json_general_test (const json_test_config_t *config)
          bson_free (selected_test);
          continue;
       }
+      bson_free (selected_test);
 
       if (bson_has_field (&test, "skipReason")) {
          fprintf (stderr,
@@ -1862,8 +1863,6 @@ run_json_general_test (const json_test_config_t *config)
          }
       }
 
-      bson_free (selected_test);
-
       uri = (config->uri_str != NULL) ? mongoc_uri_new (config->uri_str)
                                       : test_framework_get_uri ();