mosh-client: Return true for still_connecting if !network

Fixes:

    $ MOSH_KEY=foo ./mosh-client 127.0.0.1 60010
    Crypto exception: Key must be 22 letters long.
    Segmentation fault

Closes mobile-shell#189.

src/frontend/stmclient.h

@@ -53,7 +53,11 @@ class STMClient {
 
   void output_new_frame( void );
 
-  bool still_connecting( void ) { return (network->get_remote_state_num() == 0); }
+  bool still_connecting( void )
+  {
+    /* Initially, network == NULL */
+    return ( !network ) || ( network->get_remote_state_num() == 0 );
+  }
 
 public:
   STMClient( const char *s_ip, int s_port, const char *s_key, const char *predict_mode )