fix compiler warning due to possibly unknown switch enum values being handled poorly

src/bank-lib/bank_api_common.c

@@ -80,13 +80,12 @@ TALER_BANK_make_auth_header_ (const struct TALER_BANK_AuthenticationData *auth)
     authh = append (authh,
                     "X-Taler-Bank-Password",
                     auth->details.basic.password);
-    break;
-  }
     return authh;    
+  }
+  return NULL;
 }
 
 
-
 /**
  * Obtain the URL to use for an API request.
  *