chao an a8d2924c55 examples/usrsocktest: correct return value check of dup2()
DUP(2) Linux Programmer's Manual

NAME
       dup, dup2, dup3 - duplicate a file descriptor
...
RETURN VALUE
       On success, these system calls return the new file descriptor.
       On error, -1 is returned, and errno is set appropriately.

Signed-off-by: chao an <anchao@xiaomi.com>
2022-09-01 14:58:09 +08:00
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