purego: fix string argument processing in RegisterFunc

Use the correct loop variable 'val' instead of 'v' when converting string
arguments to C strings. This ensures each string argument is properly
converted rather than repeatedly using the same outer scope variable.

Author: tmc

func.go

@@ -287,7 +287,7 @@ func RegisterFunc(fptr any, cfn uintptr) {
 
 				for j, val := range args[i:] {
 					if val.Kind() == reflect.String {
-						ptr := strings.CString(v.String())
+						ptr := strings.CString(val.String())
 						keepAlive = append(keepAlive, ptr)
 						val = reflect.ValueOf(ptr)
 						args[i+j] = val