Catch exceptions by reference

Found by cppcheck.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/examples/decrypt.cc

@@ -61,7 +61,7 @@ int main( int argc, char *argv[] )
     fprintf( stderr, "Nonce = %ld\n",
 	     (long)message.nonce.val() );
     cout << message.text;
-  } catch ( CryptoException e ) {
+  } catch ( const CryptoException &e ) {
     cerr << e.text << endl;
     exit( 1 );
   }

src/examples/encrypt.cc

@@ -62,7 +62,7 @@ int main( int argc, char *argv[] )
     cerr << "Key: " << key.printable_key() << endl;
 
     cout << ciphertext;
-  } catch ( CryptoException e ) {
+  } catch ( const CryptoException &e ) {
     cerr << e.text << endl;
     exit( 1 );
   }

src/examples/ntester.cc

@@ -65,7 +65,7 @@ int main( int argc, char *argv[] )
     } else {
       n = new Transport<UserStream, UserStream>( me, remote, NULL, NULL );
     }
-  } catch ( CryptoException e ) {
+  } catch ( const CryptoException &e ) {
     fprintf( stderr, "Fatal error: %s\n", e.text.c_str() );
     exit( 1 );
   }
@@ -97,7 +97,7 @@ int main( int argc, char *argv[] )
 	    last_num = n->get_remote_state_num();
 	  }
 	}
-      } catch ( CryptoException e ) {
+      } catch ( const CryptoException &e ) {
 	fprintf( stderr, "Cryptographic error: %s\n", e.text.c_str() );
       }
     }
@@ -163,10 +163,10 @@ int main( int argc, char *argv[] )
 	if ( network_ready_to_read ) {
 	  n->recv();
 	}
-      } catch ( NetworkException e ) {
+      } catch ( const NetworkException &e ) {
 	fprintf( stderr, "%s: %s\r\n", e.function.c_str(), strerror( e.the_errno ) );
 	break;
-      } catch ( CryptoException e ) {
+      } catch ( const CryptoException &e ) {
 	fprintf( stderr, "Cryptographic error: %s\n", e.text.c_str() );
       }
     }