Support for building on MacOS clang-700.0.72

[random-ua]

Fixes following errors when building on MacOS + clang 7

nginx-dlna-module/ngx_ssdp_dlna.c:273:60: error: incompatible pointer types passing 'struct sockaddr_in *' to parameter of type 'struct sockaddr *'
      [-Werror,-Wincompatible-pointer-types]
        n = recvfrom(c->fd, in, NGX_DLNA_SSDP_UDP_SIZE, 0, &sin, &socklen);
                                                           ^~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/sys/socket.h:591:72: note: passing argument to
      parameter here
ssize_t recvfrom(int, void *, size_t, int, struct sockaddr * __restrict,
                                                                       ^
nginx-dlna-module/ngx_ssdp_dlna.c:425:70: error: incompatible pointer types passing 'struct sockaddr_in *' to parameter of type 'const struct sockaddr *'
      [-Werror,-Wincompatible-pointer-types]
        if (sendto(ngx_ssdp_dlna_socket, out, (size_t) (p - out), 0, sin,
                                                                     ^~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/sys/socket.h:597:31: note: passing argument to
      parameter here
                int, const struct sockaddr *, socklen_t) __DARWIN_ALIAS_C(sendto);

nginx-dlna-module/ngx_http_dlna.c:723:33: error: variable 'filename' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
                if (ngx_de_info(filename, &dir) == NGX_FILE_ERROR) {
                                ^~~~~~~~
nginx-dlna-module/ngx_http_dlna.c:537:47: note: initialize the variable 'filename' to silence this warning
    u_char                     type, *filename, *last, *leaf;
                                              ^
                                               = NULL