user_mgr: Fix potential out-of-bounds access

If the sysconf method returns -1, an out-of-bounds access problem
will occur when declaring the following vector:
```
std::vector<char> buffer(buflen);
```

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I1887cc418c40b01d839f9c0e25bef09a2605973b
1 file changed