patch 8.2.0078: expanding <sfile> works differently the second time

Problem:    Expanding <sfile> works differently the second time.
Solution:   Keep the expanded name when redefining a function. (closes #5425)
diff --git a/src/userfunc.c b/src/userfunc.c
index a8bc34a..95862fe 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2572,9 +2572,13 @@
 	    }
 	    else
 	    {
-		// redefine existing function
+		char_u *exp_name = fp->uf_name_exp;
+
+		// redefine existing function, keep the expanded name
 		VIM_CLEAR(name);
+		fp->uf_name_exp = NULL;
 		func_clear_items(fp);
+		fp->uf_name_exp = exp_name;
 #ifdef FEAT_PROFILE
 		fp->uf_profiling = FALSE;
 		fp->uf_prof_initialized = FALSE;