remove unused variable "err" from "clean_up"
remove unused variable "wrote" from "gridseed_detect_one"
fixed some compiler warnings: differ in signedness

fixed warnings like:
- expected ‘char *’ but argument is of type ‘unsigned char *’
- pointer targets in passing argument X of ‘xxxxx’ differ in signedness [-Wpointer-sign]
Merge pull request #1 from Mansouu/fixing-compiler-warnings

Fixing compiler warnings