sim-se: Fix crash in chdirFunc() on nonexistent directory (#277)

This commit fixes a crash in the syscall emulation of the chdir(2)
syscall, implemented by chdirFunc() in src/sim/syscall_emul.cc, when
passed a nonexistent directory. The buggy code did not check the return
value of realpath().

This patch adds code to check the return value of realpath(), and if it
is NULL (i.e., there was an error with the requested directory to change
to), propagates the error in `errno` to the application.

GitHub issue: https://github.com/gem5/gem5/issues/276

src/sim/syscall_emul.cc

@@ -959,7 +959,9 @@ chdirFunc(SyscallDesc *desc, ThreadContext *tc, VPtr<> pathname)
         tgt_cwd = path;
     } else {
         char buf[PATH_MAX];
-        tgt_cwd = realpath((p->tgtCwd + "/" + path).c_str(), buf);
+        if (!realpath((p->tgtCwd + "/" + path).c_str(), buf))
+            return -errno;
+        tgt_cwd = buf;
     }
     std::string host_cwd = p->checkPathRedirect(tgt_cwd);