patch 8.2.5151: reading beyond the end of the line with lisp indenting

Problem:    Reading beyond the end of the line with lisp indenting.
Solution:   Avoid going over the NUL at the end of the line.
diff --git a/src/indent.c b/src/indent.c
index fa177bc..f6cfe4b 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -2076,8 +2076,11 @@
 		    amount += 2;
 		else
 		{
-		    that++;
-		    amount++;
+		    if (*that != NUL)
+		    {
+			that++;
+			amount++;
+		    }
 		    firsttry = amount;
 
 		    while (VIM_ISWHITE(*that))