patch 8.2.1049: Vim9: leaking memory when using continuation line

Problem:    Vim9: leaking memory when using continuation line.
Solution:   Keep a pointer to the continuation line in evalarg_T.  Centralize
            checking for a next command.
diff --git a/src/findfile.c b/src/findfile.c
index b153ed0..190fc69 100644
--- a/src/findfile.c
+++ b/src/findfile.c
@@ -2079,7 +2079,7 @@
     char_u	*res;
 
     set_vim_var_string(VV_FNAME, ptr, len);
-    res = eval_to_string_safe(curbuf->b_p_inex, NULL,
+    res = eval_to_string_safe(curbuf->b_p_inex,
 		      was_set_insecurely((char_u *)"includeexpr", OPT_LOCAL));
     set_vim_var_string(VV_FNAME, NULL, 0);
     return res;