unix:unix-getpwuid with unknown uid produces segment violation

Describe the bug

On linux, running unix:unix-getpwuid with an unknown uid produces a segfault.

To Reproduce

Steps to reproduce the behavior:

1. Choose a uid that doesn't exist
2. (unix:unix-getpwuid uid)
3. See error:

Error in function UNIX::SIGSEGV-HANDLER:  Segmentation Violation at #x1054B2EB.
   [Condition of type SIMPLE-ERROR]

Expected behavior

This should just return NIL to indicate that there's no such uid. This is what happens on macos.

The problem appears to be that unix-getpwuid checks for the return value from getpwuid_r. However, it should check if result parameter is NULL or not instead.

Desktop (please complete the following information):

• OS: Linux