Return an error for invalid proxy URLs instead of crashing.

src/libgit2/transports/http.c

@@ -334,10 +334,19 @@ static int lookup_proxy(
 		return 0;
 	}
 
-	if (!proxy ||
-	    (error = git_net_url_parse(&transport->proxy.url, proxy)) < 0)
+	if (!proxy)
 		goto done;
 
+
+	if ((error = git_net_url_parse(&transport->proxy.url, proxy) < 0))
+		goto done;
+
+	if (!git_net_url_valid(&transport->proxy.url)) {
+		git_error_set(GIT_ERROR_HTTP, "invalid proxy url: %s", proxy);
+		error = GIT_EINVALIDSPEC;
+		goto done;
+	}
+
 	*out_use = true;
 
 done:

tests/libgit2/online/clone.c

@@ -968,6 +968,19 @@ static int proxy_cert_cb(git_cert *cert, int valid, const char *host, void *payl
 	return valid ? 0 : GIT_ECERTIFICATE;
 }
 
+void test_online_clone__proxy_invalid_url(void)
+{
+	g_options.fetch_opts.proxy_opts.type = GIT_PROXY_SPECIFIED;
+	g_options.fetch_opts.proxy_opts.credentials = proxy_cred_cb;
+	g_options.fetch_opts.proxy_opts.certificate_check = proxy_cert_cb;
+
+	g_options.fetch_opts.proxy_opts.url = "noschemeorport";
+	cl_git_fail_with(GIT_EINVALIDSPEC, git_clone(&g_repo, "http://github.com/libgit2/TestGitRepository", "./foo", &g_options));
+
+	g_options.fetch_opts.proxy_opts.url = "noscheme:8080";
+	cl_git_fail_with(GIT_EINVALIDSPEC, git_clone(&g_repo, "http://github.com/libgit2/TestGitRepository", "./foo", &g_options));
+}
+
 void test_online_clone__proxy_credentials_request(void)
 {
 	git_str url = GIT_STR_INIT;