Fixes #181 - Cast to const char * when calling strlen() in string.c

:%s/strlen(\s*\([^() ]+\)\s*)/strlen( ( const char * )\1 )/g

This cast should hopefully suppress those static analysis warnings
regarding strlen()'s parameter.
Merge pull request #181 from caitp/issue-181

string.c: case byte * to const char * when calling strlen (prevent static-analysis warnings)

It's essentially a style change, pretty trivial. Won't functionally break anything.