Fix bug of n in function of ngx_utf8_length

[liu yunbin]

# HG changeset patch
# User Yunbin Liu yunbinliu at outlook.com
# Date 1554925873 -28800
#      Thu Apr 11 03:51:13 2019 +0800
# Node ID 228b945cf5f8c30356fc5760f696e49545075f00
# Parent  a6e23e343081b79eb924da985a414909310aa7a3
Fix bug of n in function of ngx_utf8_length

diff -r a6e23e343081 -r 228b945cf5f8 src/core/ngx_string.c
--- a/src/core/ngx_string.c Tue Apr 09 16:00:30 2019 +0300
+++ b/src/core/ngx_string.c Thu Apr 11 03:51:13 2019 +0800
@@ -1369,6 +1369,7 @@
 {
     u_char  c, *last;
     size_t  len;
+    u_char *current_point;

     last = p + n;

@@ -1378,13 +1379,16 @@

         if (c < 0x80) {
             p++;
+            n--;
             continue;
         }

+        current_point = p;
         if (ngx_utf8_decode(&p, n) > 0x10ffff) {
             /* invalid UTF-8 */
             return n;
         }
+        n -= p - current_point;
     }

     return len;
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