checker: do not allow nil sum types init

vlib/builtin/linux_bare/builtin_bare.v

@@ -0,0 +1,60 @@
+module builtin
+
+// called by the generated main/init
+fn init() {
+}
+
+pub fn isnil(p voidptr) bool {
+	return p == 0
+}
+
+pub fn print(s string) {
+	sys_write(1, s.str, u64(s.len))
+}
+
+pub fn println(s string) {
+	print(s)
+	print('\n')
+}
+
+pub fn panic(s string) {
+	eprint('V panic: ')
+	eprintln(s)
+	sys_exit(1)
+}
+
+// replaces panic when -debug arg is passed
+fn panic_debug(line_no int, file string, mod string, fn_name string, s string) {
+	eprintln('================ V panic ================')
+	eprint('   module: ')
+	eprintln('mod')
+	eprint(' function: ')
+	eprint(fn_name)
+	eprintln('()')
+	eprintln('     file: ')
+	eprintln(file)
+	// println('     line: ${line_no}')
+	eprint('  message: ')
+	eprintln(s)
+	eprintln('=========================================')
+	sys_exit(1)
+}
+
+pub fn eprint(s string) {
+	if isnil(s.str) {
+		panic('eprint(NIL)')
+	}
+	sys_write(2, s.str, u64(s.len))
+}
+
+pub fn eprint_ln(s string) {
+	eprint(s)
+	eprint('\n')
+}
+
+pub fn eprintln(s string) {
+	if isnil(s.str) {
+		panic('eprintln(NIL)')
+	}
+	eprint_ln(s)
+}