[whatwg/url] Path setter jumps off the leading edge of input (#480)

When the path setter is called, the basic parser is overridden to the *path start state*. But per either 1.3 or 4.2 for that state, the pointer can be decremented:

> If c is neither U+002F (/) nor U+005C (\), then decrease pointer by one.

> If c is not U+002F (/), then decrease pointer by one.

This is ill-defined when the pointer is to the start of the input.

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