Valgrind: fix "definitely lost" memory from get_random_string() - test-address

==88262== 6 bytes in 1 blocks are definitely lost in loss record 5 of 2,117
==88262==    at 0x4848A13: calloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==88262==    by 0x503C550: g_malloc0 (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7600.1)
==88262==    by 0x10B043: get_random_string_without (test-stuff.c:312)
==88262==    by 0x10B0C0: get_random_string (test-stuff.c:333)
==88262==    by 0x10A6E0: test_string_fcn (test-address.c:79)
==88262==    by 0x10A694: test_address (test-address.c:69)
==88262==    by 0x10A7C5: main (test-address.c:94)

+ 7 more

libgnucash/engine/test/test-address.c

@@ -76,12 +76,13 @@ test_string_fcn (GncAddress *address, const char *message,
                  void (*set) (GncAddress *, const char *str),
                  const char * (*get)(const GncAddress *))
 {
-    char const *str = get_random_string ();
+    char *str = get_random_string ();
 
     do_test (!gncAddressIsDirty (address), "test if start dirty");
     set (address, str);
     do_test (gncAddressIsDirty (address), "test dirty later");
     do_test (g_strcmp0 (get (address), str) == 0, message);
+    g_free (str);
     gncAddressClearDirty (address);
 }