patch 8.2.3239: Vim9: no error using heredoc for a number variable

Problem:    Vim9: no error using heredoc for a number variable.
Solution:   Add a type check. (closes #8627)
diff --git a/src/evalvars.c b/src/evalvars.c
index 534163c..dc747f7 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -817,6 +817,7 @@
     else if (expr[0] == '=' && expr[1] == '<' && expr[2] == '<')
     {
 	list_T	*l;
+	long	cur_lnum = SOURCING_LNUM;
 
 	// HERE document
 	l = heredoc_get(eap, expr + 3, FALSE);
@@ -825,6 +826,8 @@
 	    rettv_list_set(&rettv, l);
 	    if (!eap->skip)
 	    {
+		// errors are for the assignment, not the end marker
+		SOURCING_LNUM = cur_lnum;
 		op[0] = '=';
 		op[1] = NUL;
 		(void)ex_let_vars(eap->arg, &rettv, FALSE, semicolon, var_count,