Fix a bug: if the less file end line is comments, the lessc command option "modify-var" will have no effect.

[chenboxiang]

Here is an example: the test.less file source code:

@height: 20px;
.cls {
    height: @height;
}
// comments

now execute the command:

bin/lessc test.less --modify-var="height=30px"

the expected result is the @height will transform to 30px, but it is still 20px.

[lukeapage]

thanks