Win32: improved ngx_mutex_init() stub (ticket #138).

This allows to run nginx with "master_process off" under Windows.

src/os/win32/ngx_thread.c

diff --git a/src/os/win32/ngx_thread.c b/src/os/win32/ngx_thread.c
index 84c9d51..1bdd6d2 100644
--- a/src/os/win32/ngx_thread.c
+++ b/src/os/win32/ngx_thread.c
@@ -72,7 +72,18 @@
 ngx_mutex_t *
 ngx_mutex_init(ngx_log_t *log, ngx_uint_t flags)
 {
-    return (ngx_mutex_t *) 1;
+    ngx_mutex_t  *m;
+
+    m = ngx_alloc(sizeof(ngx_mutex_t), log);
+    if (m == NULL) {
+        return NULL;
+    }
+
+    m->log = log;
+
+    /* STUB */
+
+    return m;
 }