How to preallocate cstring for FFI call that fills it up

[PMunch]

When you have a C library that populates a string it is good practice to use 
`setLen` on the string afterwards. Most of these APIs returns the amount of 
bytes actually written (in this case by the `length` parameter to 
`glGetActiveUniform` as long as it is not `nil` as in your case. This reports 
the length without the null terminator, so it should strip this for you 
automatically.