Missing error checks for the `ngx_http_lua_coroutine_create_helper` function call

The clang static analyzer found a call of ngx_http_lua_coroutine_create_helper missing error handling:

src/ngx_http_lua_coroutine.c:97:20: warning: Access to field 'is_wrap' results in a dereference of a null pointer (loaded from variable 'coctx')
    coctx->is_wrap = 1;
    ~~~~~          ^
1 warning generated.

The corresponding code snippets is like this:

 95     ngx_http_lua_coroutine_create_helper(L, r, ctx, &coctx);
 96
 97     coctx->is_wrap = 1;

git blame shows the following:

9a892b8d5 (Thibault Charbonnier   2018-01-17 22:20:44 -0800  95)     ngx_http_lua_coroutine_create_helper(L, r, ctx, &coctx);
9a892b8d5 (Thibault Charbonnier   2018-01-17 22:20:44 -0800  96)
9a892b8d5 (Thibault Charbonnier   2018-01-17 22:20:44 -0800  97)     coctx->is_wrap = 1;