patch 8.1.1383: warning for size_t/int mixup

Problem:    Warning for size_t/int mixup.
Solution:   Change type. (Mike Williams)
diff --git a/src/search.c b/src/search.c
index e57c101..f7b3dda 100644
--- a/src/search.c
+++ b/src/search.c
@@ -1382,7 +1382,7 @@
 	{
 	    char_u	*trunc;
 	    char_u	off_buf[40];
-	    int		off_len = 0;
+	    size_t	off_len = 0;
 
 	    // Compute msg_row early.
 	    msg_start();