Protesting and demonstrating is not a riot.

Under federal law, a riot is a public disturbance involving an act of violence by one or more persons assembled in a group of at least three people. Inciting a riot applies to a person who organizes, encourages, or participates in a riot. It can apply to one who urges or instigates others to riot. According to 18 USCS § 2102 "to incite a riot", or "to organize, promote, encourage, participate in, or carry on a riot", includes, but is not limited to, urging or instigating other persons to riot, but shall not be deemed to mean the mere oral or written (1) advocacy of ideas or (2) expression of belief, not involving advocacy of any act or acts of violence or assertion of the rightness of, or the right to commit, any such act or acts.”

Booing is not a riot.

Those that support candidates need to discern the different between protesting and rioting. It is not the same thing.

1990: United States v. Eichman (496 U.S. 310) (click here) — Passage of the Flag Protection Act resulted in a number of flag burning incidents protesting the new law. The Supreme Court overturned several flag burning convictions brought under the Flag Protection Act of 1989. The Court held that notwithstanding Congress' effort to adopt a more content neutral law, the federal law continued to be principally aimed at limiting symbolic speech.

A person can legally burn the US flag, but, they might be susceptible to a misdemeanor of starting a fire.

Then there is the 'maintenance flag burning.' 

If one is worried about being charged with a misdemeanor of flag burning while protesting; perhaps it would prove less peril if burning an old flag.