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/quickfix.c b/src/quickfix.c
index ba54fab..3bd0f75 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -7680,7 +7680,7 @@
 
     // Evaluate the expression.  When the result is a string or a list we can
     // use it to fill the errorlist.
-    tv = eval_expr(eap->arg, &eap->nextcmd);
+    tv = eval_expr(eap->arg, eap);
     if (tv != NULL)
     {
 	if ((tv->v_type == VAR_STRING && tv->vval.v_string != NULL)