Don't check nibbling mass if nibbling is disabled

This is a tiny fix that allows the code to run when gas particle nibbling is disabled. The code checks that the minimum gas particle mass for nibbling is within a factor of 10^5 of the subgrid seed mass. This check still happens when use_nibbling=0, even though min_gas_mass_for_nibbling_Msun has not been set, and the code crashes.

This merge request just moves the check into the 'if(use_nibbling)' block.