linux: fix signedness issue in uv_exepath()
linux: fix signedness issue in uv_exepath()

readlink() returns -1 on error. The <= 0 check failed to catch that because the
return value was implicitly cast to size_t, which is unsigned.

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