patch 8.2.1847: Vim9: using negative value for unsigned type

Problem:    Vim9: using negative value for unsigned type.
Solution:   Use zero instead of -1.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index ad9b9df..0f29ceb 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -1840,7 +1840,7 @@
 	return -1;
     if (sid == current_sctx.sc_sid)
     {
-	sallvar_T *sav = find_script_var(name, (size_t)-1, cctx);
+	sallvar_T *sav = find_script_var(name, 0, cctx);
 
 	if (sav == NULL)
 	    return -2;