builtin/submodule--helper: fix leaking error buffer

Fix leaking error buffer when `compute_alternate_path()` fails.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1621,6 +1621,8 @@ static int add_possible_reference_from_superproject(
 				; /* nothing */
 			}
 		}
+
+		strbuf_release(&err);
 		strbuf_release(&sb);
 	}
 

t/t7408-submodule-reference.sh

@@ -4,6 +4,8 @@
 #
 
 test_description='test clone --reference'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 base_dir=$(pwd)