unix: fix uv_spawn() NULL pointer deref on ENOMEM
unix: fix uv_spawn() NULL pointer deref on ENOMEM

In the cleanup-after-error section of uv_spawn(), check that the pointer
is non-NULL - we might end up in said section due to a malloc() failure.

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