fix building on platforms with non-supported atomic operations

src/core/ngx_shmtx.h

diff --git a/src/core/ngx_shmtx.h b/src/core/ngx_shmtx.h
index 7d4c09e..e6fb6aa 100644
--- a/src/core/ngx_shmtx.h
+++ b/src/core/ngx_shmtx.h
@@ -58,6 +58,8 @@
     }
 
     ngx_log_abort(err, ngx_trylock_fd_n " failed");
+
+    return 0;
 }