CL 497075 refactored NewFile to unconditionally dereference the file
returned by newFile. However, newFile can return nil if passed a
negative FD, which now causes a crash.

Resolve this by moving the invalid check earlier in NewFile, which also
lets us avoid a useless fcntl syscall on a negative FD.

Since we convert to int to check sign, adjust newFile to take an int
rather than uintptr, which cleans up a lot of conversions.

Fixes #60406

Change-Id: I382a74e22f1cc01f7a2dcf1ff4efca6a79c4dd57
Reviewed-on: https://go-review.googlesource.com/c/go/+/497877


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