Fix potential SIGSEGV in SbSocketWaiterPrivate initialization (#8464)

Replace SB_DCHECK with SB_CHECK when validating server_socket in
GetSocketPipe().

SB_DCHECK is compiled out in Release builds, leading to undefined
behavior (null pointer dereference) when AcceptBySpinning() fails or
times out. This UB manifests as SIGSEGV crash at line 146 when accessing
server_socket_->socket_fd.

SB_CHECK ensures validation in all build configurations and provides
clear error message on failure instead of unpredictable crash.

Author: msolianko

starboard/shared/libevent/socket_waiter_internal.cc

@@ -113,7 +113,7 @@ void GetSocketPipe(SbSocket* client_socket, SbSocket* server_socket) {
 
   // Spin until the accept happens (or we get impatient).
   *server_socket = AcceptBySpinning(listen_socket, kTimeoutUsec);
-  SB_DCHECK(SbSocketIsValid(*server_socket));
+  SB_CHECK(SbSocketIsValid(*server_socket));
 
   result = SbSocketDestroy(listen_socket);
   SB_DCHECK(result);