patch 8.0.1416: crash when searching for a sentence

Problem:    Crash when searching for a sentence.
Solution:   Return NUL when getting character at MAXCOL. (closes #2468)

src/misc1.c

@@ -2650,8 +2650,12 @@ del_lines(
     int
 gchar_pos(pos_T *pos)
 {
-    char_u	*ptr = ml_get_pos(pos);
+    char_u	*ptr;
 
+    /* When searching columns is sometimes put at the end of a line. */
+    if (pos->col == MAXCOL)
+	return NUL;
+    ptr = ml_get_pos(pos);
 #ifdef FEAT_MBYTE
     if (has_mbyte)
 	return (*mb_ptr2char)(ptr);