Improvement to basename.c

While it has no real performance improvements, 
```
cyphar@fresh:~/Projects/aliceinuserland$ time ./a src/false.c # my code
false.c

real	0m0.001s
user	0m0.000s
sys	0m0.000s
cyphar@fresh:~/Projects/aliceinuserland$ time ./b src/false.c # your code
false.c

real	0m0.001s
user	0m0.000s
sys	0m0.000s
```
this code is (in my opinion) far more straight-forward.

BTW, I love your work. I have been thinking about making a GNU-free OS for a very long time. Keep coding.