Avoid crashing on invalid GitHub hostname

Fixes #2022

features/create.feature

@@ -274,3 +274,28 @@ Feature: hub create
       < Location: http://disney.com
       {"full_name":"mislav/dotfiles"}\n
       """
+
+  Scenario: Create Enterprise repo
+    Given I am "nsartor" on git.my.org with OAuth token "FITOKEN"
+    Given the GitHub API server:
+      """
+      post('/api/v3/user/repos', :host_name => 'git.my.org') {
+        assert :private => false
+        status 201
+        json :full_name => 'nsartor/dotfiles'
+      }
+      """
+    And $GITHUB_HOST is "git.my.org"
+    When I successfully run `hub create`
+    Then the url for "origin" should be "git@git.my.org:nsartor/dotfiles.git"
+    And the output should contain exactly "https://git.my.org/nsartor/dotfiles\n"
+
+  Scenario: Invalid GITHUB_HOST
+    Given I am "nsartor" on {} with OAuth token "FITOKEN"
+    And $GITHUB_HOST is "{}"
+    When I run `hub create`
+    Then the exit status should be 1
+    And the stderr should contain exactly:
+      """
+      invalid hostname: "{}"\n
+      """

github/client.go

@@ -992,8 +992,10 @@ func (client *Client) apiClient() *simpleClient {
 }
 
 func (client *Client) absolute(host string) *url.URL {
-	u, _ := url.Parse("https://" + host + "/")
-	if client.Host != nil && client.Host.Protocol != "" {
+	u, err := url.Parse("https://" + host + "/")
+	if err != nil {
+		panic(err)
+	} else if client.Host != nil && client.Host.Protocol != "" {
 		u.Scheme = client.Host.Protocol
 	}
 	return u

github/config.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"fmt"
 	"io/ioutil"
+	"net/url"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -40,6 +41,13 @@ func (c *Config) PromptForHost(host string) (h *Host, err error) {
 	token := c.DetectToken()
 	tokenFromEnv := token != ""
 
+	if host != GitHubHost {
+		if _, e := url.Parse("https://" + host); e != nil {
+			err = fmt.Errorf("invalid hostname: %q", host)
+			return
+		}
+	}
+
 	h = c.Find(host)
 	if h != nil {
 		if h.User == "" {