About 1% of all amino acids (which proteins are made of) in your body are in free form. For example inside the cells or in the blood. So that pool can be used for muscle building.
But you also should not forget that muscle building is a pretty slow process and it's not like all your muscle growth will happen at night. So the amount of protein needed for that won't be that much. Especially compared to the amount of protein needed for other purposes.
Proteins basically run the whole body. Or they are the whole body. The immune system, all enzymes, structural proteins, hormones (e.g. insulin) etc.etc. So you can safely assume that the body has a "storage space" for amino acids like he has for glucose and fat.
So as a normal, healthy person I wouldn't worry too much about getting a protein deficit only because you don't eat protein for half a day.