os: fix os.real_path on Windows (#8822)

vlib/builtin/cfns.c.v

@@ -275,6 +275,8 @@ fn C._wgetcwd() int
 
 fn C._fullpath() int
 
+fn C.GetFullPathName(voidptr, u32, voidptr, voidptr) u32
+
 fn C.GetCommandLine() voidptr
 
 fn C.LocalFree()

vlib/os/os_c.v

@@ -744,23 +744,32 @@ pub fn getwd() string {
 // NB: this particular rabbit hole is *deep* ...
 [manualfree]
 pub fn real_path(fpath string) string {
-	mut fullpath := vcalloc(max_path_len)
+	mut fullpath := byteptr(0)
 	defer {
 		unsafe { free(fullpath) }
 	}
-	mut ret := charptr(0)
+
 	$if windows {
-		ret = charptr(C._fullpath(charptr(fullpath), charptr(fpath.str), max_path_len))
+		fullpath = unsafe { &u16(vcalloc(max_path_len * 2)) }
+		// TODO: check errors if path len is not enough
+		ret := C.GetFullPathName(fpath.to_wide(), max_path_len, fullpath, 0)
 		if ret == 0 {
 			return fpath
 		}
 	} $else {
-		ret = charptr(C.realpath(charptr(fpath.str), charptr(fullpath)))
+		fullpath = vcalloc(max_path_len)
+		ret := charptr(C.realpath(charptr(fpath.str), charptr(fullpath)))
 		if ret == 0 {
 			return fpath
 		}
 	}
-	res := unsafe { fullpath.vstring() }
+
+	mut res := ''
+	$if windows {
+		res = unsafe { string_from_wide(fullpath) }
+	} $else {
+		res = unsafe { fullpath.vstring() }
+	}
 	nres := normalize_drive_letter(res)
 	cres := nres.clone()
 	return cres

vlib/os/os_test.v

@@ -271,6 +271,10 @@ fn test_cp_all() {
 	os.cp_all('ex', './', true) or { panic(err) }
 }
 
+fn test_realpath() {
+	assert os.real_path('') == ''
+}
+
 fn test_tmpdir() {
 	t := os.temp_dir()
 	assert t.len > 0