patch 8.2.4968: reading past end of the line when C-indenting

Problem:    Reading past end of the line when C-indenting.
Solution:   Check for NUL.
diff --git a/src/cindent.c b/src/cindent.c
index ec88036..2d47e64 100644
--- a/src/cindent.c
+++ b/src/cindent.c
@@ -89,7 +89,7 @@
 		while (vim_isdigit(p[i - 1]))   // '\000'
 		    ++i;
 	    }
-	    if (p[i] == '\'')		    // check for trailing '
+	    if (p[i - 1] != NUL && p[i] == '\'')    // check for trailing '
 	    {
 		p += i;
 		continue;