Fix ErrorWithPosition panic when less than two lines

[xoltia]

Currently, it is possible for ParserError.ErrorWithPosition to cause a panic for some input.
You can reproduce the panic using this example:

v := &struct{ String string }{}
_, err := toml.Decode(`string = "test`, v)
parseErr := err.(toml.ParseError)
_ = parseErr.ErrorWithUsage()

I've added a fix and a new test case which covers this error.

[arp242]

This is line 1, not 0.

[xoltia]

Sorry, I didn't notice that. I've updated the test and believe I found where the line number was being incorrectly set to 0.

[arp242]

Merged via 4664468; thanks!