CDRIVER-5878 Free child on failed call to bson_append_array_builder_begin (#1849)

* add failing test

* free `child` on failure in `bson_append_array_builder_begin`

Author: kevinAlbs

src/libbson/src/bson/bson.c

@@ -2971,7 +2971,12 @@ bson_append_array_builder_begin (bson_t *bson, const char *key, int key_length,
    BSON_ASSERT_PARAM (key);
    BSON_ASSERT_PARAM (child);
    *child = bson_array_builder_new ();
-   return bson_append_array_begin (bson, key, key_length, &(*child)->bson);
+   bool ok = bson_append_array_begin (bson, key, key_length, &(*child)->bson);
+   if (!ok) {
+      bson_array_builder_destroy (*child);
+      *child = NULL;
+   }
+   return ok;
 }
 
 bool

src/libbson/tests/test-bson.c

@@ -3196,6 +3196,16 @@ test_bson_array_builder (void)
          bson_array_builder_destroy (bab);
       }
    }
+
+   // A failure in bson_append_array_builder_begin does not allocate.
+   {
+      bson_t b = BSON_INITIALIZER;
+      bson_array_builder_t *child;
+      bool ok = bson_append_array_builder_begin (&b, "has_embedded_null\0", strlen ("has_embedded_null") + 1, &child);
+      ASSERT (!ok);
+      bson_destroy (&b);
+      // Not necessary to free `child`.
+   }
 }
 
 void