patch 8.2.4182: memory leak when evaluating 'diffexpr'

Problem:    Memory leak when evaluating 'diffexpr'.
Solution:   Use free_tv() instead of clear_tv().
diff --git a/src/evalvars.c b/src/evalvars.c
index a1d2b9b..9dbdc75 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -429,7 +429,7 @@
 
     // errors are ignored
     tv = eval_expr(p_dex, NULL);
-    clear_tv(tv);
+    free_tv(tv);
 
     set_vim_var_string(VV_FNAME_IN, NULL, -1);
     set_vim_var_string(VV_FNAME_NEW, NULL, -1);