"while(null) break;" panics

[mstoykov]

The following code panics:

while(null) {
  break;
}

the same goes if null is "".

panic: runtime error: index out of range [0] with length 0 [recovered]
        panic: runtime error: index out of range [0] with length 0

goroutine 6 [running]:
testing.tRunner.func1.1(0x828b60, 0xc000016b80)
        testing/testing.go:1003 +0x30d
testing.tRunner.func1(0xc0000ca360)
        testing/testing.go:1006 +0x3f9
panic(0x828b60, 0xc000016b80)
        runtime/panic.go:969 +0x166
github.com/dop251/goja.(*compiler).leaveBlock(0xc0000a0b00)
        github.com/dop251/goja/compiler.go:89 +0x1ee
github.com/dop251/goja.(*compiler).compileLabeledWhileStatement(0xc0000a0b00, 0xc000010de0, 0x7aab7e8e04f0c400, 0x0, 0x0)
        github.com/dop251/goja/compiler_stmt.go:426 +0x26e
github.com/dop251/goja.(*compiler).compileWhileStatement(0xc0000a0b00, 0xc000010de0, 0x203000)
        github.com/dop251/goja/compiler_stmt.go:376 +0x46
github.com/dop251/goja.(*compiler).compileStatement(0xc0000a0b00, 0x919920, 0xc000010de0, 0x919300)
        github.com/dop251/goja/compiler_stmt.go:36 +0x609
github.com/dop251/goja.(*compiler).compileStatements(0xc0000a0b00, 0xc0000ef680, 0x5, 0x8, 0x203001)
        github.com/dop251/goja/compiler_stmt.go:724 +0x105
github.com/dop251/goja.(*compiler).compile(0xc0000a0b00, 0xc0000a0ac0)
        github.com/dop251/goja/compiler.go:265 +0x1b0