Avoid possible overflow in OS_Unix readlink

Also fix [-Wunused-result]

drivers/unix/os_unix.cpp

@@ -487,9 +487,11 @@ String OS_Unix::get_executable_path() const {
 	//fix for running from a symlink
 	char buf[256];
 	memset(buf, 0, 256);
-	readlink("/proc/self/exe", buf, sizeof(buf));
+	ssize_t len = readlink("/proc/self/exe", buf, sizeof(buf));
 	String b;
-	b.parse_utf8(buf);
+	if (len > 0) {
+		b.parse_utf8(buf, len);
+	}
 	if (b == "") {
 		WARN_PRINT("Couldn't get executable path from /proc/self/exe, using argv[0]");
 		return OS::get_executable_path();