Fix dead-assignment warnings in test_config.c

Found by scan-build.

src/test/test_config.c

@@ -5783,6 +5783,7 @@ test_config_extended_fmt(void *arg)
   tt_str_op(lp->value, OP_EQ, "is back here");
   tt_int_op(lp->command, OP_EQ, CONFIG_LINE_NORMAL);
   lp = lp->next;
+  tt_assert(!lp);
   config_free_lines(lines);
 
   /* Try with the "extended" flag enabled. */
@@ -5809,6 +5810,7 @@ test_config_extended_fmt(void *arg)
   tt_str_op(lp->value, OP_EQ, "");
   tt_int_op(lp->command, OP_EQ, CONFIG_LINE_CLEAR);
   lp = lp->next;
+  tt_assert(!lp);
 
  done:
   config_free_lines(lines);