Reverted incorrect change in internal md5 (part of r3928).

src/core/ngx_md5.c

diff --git a/src/core/ngx_md5.c b/src/core/ngx_md5.c
index 09a9399..440c75b 100644
--- a/src/core/ngx_md5.c
+++ b/src/core/ngx_md5.c
@@ -47,7 +47,8 @@
             return;
         }
 
-        data = ngx_cpymem(&ctx->buffer[used], data, free);
+        ngx_memcpy(&ctx->buffer[used], data, free);
+        data = (u_char *) data + free;
         size -= free;
         (void) ngx_md5_body(ctx, ctx->buffer, 64);
     }