Terminate variables with string literals with a zero.

examples/print.g

@@ -1,12 +1,14 @@
+JUMP run
 VARB msg "hello world"
 
 .run
-SETX msg 	; set X register to the address of msg
+SETX msg 	    ; set X register to the address of msg
+JUMP print
 
 .print
-MOVE *X -> A		; move the value of address in X to A
-OUTA		; print A
-INCX		; increment address stored in X
-JANZ print	; jump to label 'done' if A = 0
+MOVE *X -> A    ; move the value at address in X to A
+OUTA		    ; print A
+INCX		    ; increment address stored in X
+JANZ print	    ; jump to label 'done' if A = 0
 HALT
 

gmachine.go

@@ -293,7 +293,7 @@ func Assemble(reader io.Reader) ([]Word, error) {
 			case ast.IntegerLiteral:
 				program = append(program, Word(operand.Value))
 			case ast.StringLiteral:
-				strSlice := make([]Word, len(operand.Value))
+				strSlice := make([]Word, len(operand.Value)+1)
 				for i, c := range operand.Value {
 					strSlice[i] = Word(c)
 				}

gmachine_test.go

@@ -914,12 +914,18 @@ HALT
 func TestVARB_DeclaresAStringVariableInMemory(t *testing.T) {
 	t.Parallel()
 	g := gmachine.New(nil)
-	err := assembleAndRunFromString(g, `VARB msg "hello world"`)
+	err := assembleAndRunFromString(g, `
+JUMP start
+VARB msg "hello world"
+.start
+HALT
+`)
 	if err != nil {
 		t.Fatal("didn't expect an error:", err)
 	}
-	want := []gmachine.Word{'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'}
-	got := g.Memory[int(g.MemOffset) : int(g.MemOffset)+len(want)]
+	want := []gmachine.Word{'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', 0}
+	offsetStart := int(g.MemOffset) + 2
+	got := g.Memory[offsetStart : offsetStart+len(want)]
 	if !cmp.Equal(want, got) {
 		t.Error(cmp.Diff(want, got))
 	}