windows: Fix an infinite loop in uv_spawn

The PATH-parsing code for windows erroneously contained an infinite
loop when the PATH started with a leading semicolon. Each iteration of
the loop usually bumped over the separator, but if the first character
was a semicolon then it would never skip it, causing the infinite
loop.

Closes #909
test: Close process handles when spawn fails

This should avoid assertions tripping in debug mode.