patch 8.2.1159: Vim9: no error for missing space after a comma

Problem:    Vim9: no error for missing space after a comma.
Solution:   Check for white space.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 07aacb3..0aae8f2 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3060,7 +3060,14 @@
 	    break;
 	++count;
 	if (*p == ',')
+	{
 	    ++p;
+	    if (*p != ']' && !IS_WHITE_OR_NUL(*p))
+	    {
+		semsg(_(e_white_after), ",");
+		return FAIL;
+	    }
+	}
 	whitep = p;
 	p = skipwhite(p);
     }