packer: don't crash if arg is empty [GH-832]

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 BUG FIXES:
 
+* core: Fix crash case if blank parameters are given to Packer. [GH-832]
 * builders/docker: user variables work properly. [GH-777]
 * builder/virtualbox,vmware: iso\_checksum is not required if the
   checksum type is "none"

packer/environment.go

@@ -221,7 +221,7 @@ func (e *coreEnvironment) Cli(args []string) (result int, err error) {
 
 	// Trim up to the command name
 	for i, v := range args {
-		if v[0] != '-' {
+		if len(v) > 0 && v[0] != '-' {
 			args = args[i:]
 			break
 		}

packer/environment_test.go

@@ -198,10 +198,17 @@ func TestEnvironment_Cli_CallsRun(t *testing.T) {
 func TestEnvironment_DefaultCli_Empty(t *testing.T) {
 	defaultEnv := testEnvironment()
 
+	// Test with no args
 	exitCode, _ := defaultEnv.Cli([]string{})
 	if exitCode != 1 {
 		t.Fatalf("bad: %d", exitCode)
 	}
+
+	// Test with only blank args
+	exitCode, _ = defaultEnv.Cli([]string{""})
+	if exitCode != 1 {
+		t.Fatalf("bad: %d", exitCode)
+	}
 }
 
 func TestEnvironment_DefaultCli_Help(t *testing.T) {