Executive compensation topping $38.2 million is nothing to sneeze at . Unless you lose it. That's what United Therapeutics' Martine Rothblatt is facing. The chairman and chief executive of the Silver Spring-based life sciences company was among the highest-compensated executives in the nation, pulling down that number in 2013. That was a leap up from $5 million the year before, thanks to a surge in the stock price for United, which specializes in treating respiratory and cardiovascular diseases. Shareholders…