change surplus for() to while()

src/core/ngx_string.c

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index d73cb86..0e33e38 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -30,12 +30,15 @@
         return dst;
     }
 
-    for ( /* void */ ; --n; dst++, src++) {
+    while (--n) {
         *dst = *src;
 
         if (*dst == '\0') {
             return dst;
         }
+
+        dst++;
+        src++;
     }
 
     *dst = '\0';