Fix ErrorWithPosition panic when less than two lines

Would panic if the TOML had just one line without a newline at the end
of the file.

Author: xoltia

error_test.go

@@ -245,6 +245,19 @@ func TestParseError(t *testing.T) {
             |         15:04:05.856018510
 			`,
 		},
+
+		{
+			&struct{ String string }{},
+			`string = "test`,
+			`
+            | toml: error: unexpected EOF; expected '"'
+			|
+			| At line 1, column 14:
+			|
+			|       1 | string = "test
+			|                        ^
+			`,
+		},
 	}
 
 	prep := func(s string) string {

lex.go

@@ -275,7 +275,9 @@ func (lx *lexer) errorPos(start, length int, err error) stateFn {
 func (lx *lexer) errorf(format string, values ...any) stateFn {
 	if lx.atEOF {
 		pos := lx.getPos()
-		pos.Line--
+		if lx.pos >= 1 && lx.input[lx.pos-1] == '\n' {
+			pos.Line--
+		}
 		pos.Len = 1
 		pos.Start = lx.pos - 1
 		lx.items <- item{typ: itemError, pos: pos, err: fmt.Errorf(format, values...)}