unix: fix assert on signal pipe overflow
unix: fix assert on signal pipe overflow

An incorrect assert() statement was causing libuv to crash when writing
to an internal signal pipe would result in EAGAIN/EWOULDBLOCK.

This commit doesn't solve the underlying issue that the signal pipe can
overflow.

This should fix joyent/node#5538

unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2