patch 8.2.1283: Vim9: error for misplaced -> lacks argument

Problem:    Vim9: error for misplaced -> lacks argument.
Solution:   Use the pointer before it was advanced.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 0fe0b16..3be8733 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3761,6 +3761,8 @@
 	}
 	else if (*p == '-' && p[1] == '>')
 	{
+	    char_u *pstart = p;
+
 	    if (generate_ppconst(cctx, ppconst) == FAIL)
 		return FAIL;
 
@@ -3786,7 +3788,7 @@
 		p = *arg;
 		if (!eval_isnamec1(*p))
 		{
-		    semsg(_(e_trailing_arg), p);
+		    semsg(_(e_trailing_arg), pstart);
 		    return FAIL;
 		}
 		if (ASCII_ISALPHA(*p) && p[1] == ':')