eliminate redundant return line

[dmalan]

https://github.com/cs50/lectures/blob/master/2017/fall/3/binary0.c#L66

Though clang complains with

binary0.c:66:1: error: control may reach end of non-void function
      [-Werror,-Wreturn-type]

without.

[swatisingh45]

Hey! @dmalan, the link says page not found.