[Coverity 1427588] Dereference null return value in linux/dirs.c

If the function actually returns a null value, a null pointer dereference will occur.

In config_GetLibDirRaw: Return value of function which returns null is dereferenced without checking (CWE-476)

     10. returned_null: strrchr returns NULL (checked 23 out of 24 times). [Note: The source code implementation of the function has been overridden by a builtin model.]
     11. var_assigned: Assigning: file = NULL return value from strrchr.
63        char *file = strrchr (line, '/');
     12. Condition end == NULL, taking false branch.
64        if (end == NULL)
65            continue;
    
CID 1427588: (#1 of 1): Dereference null return value (NULL_RETURNS)
13. dereference: Dereferencing file, which is known to be NULL.
66        *file = '\0';

Should probably have been a test on file instead of end in 79399150.