Fix crash when calling openat with sandbox enabled #25440

The seccomp rule for the openat syscall checks for the AT_FDCWD
constant. Because this constant is usually a negative value, a
cast to unsigned int is necessary to make sure it does not get
converted to uint64_t used by seccomp.

More info on:
https://github.com/seccomp/libseccomp/issues/69#issuecomment-273805980

src/common/sandbox.c

@@ -469,7 +469,7 @@ allow_file_open(scmp_filter_ctx ctx, int use_openat, const char *file)
 {
   if (use_openat) {
     return seccomp_rule_add_2(ctx, SCMP_ACT_ALLOW, SCMP_SYS(openat),
-                              SCMP_CMP_STR(0, SCMP_CMP_EQ, AT_FDCWD),
+                              SCMP_CMP(0, SCMP_CMP_EQ, (unsigned int)AT_FDCWD),
                               SCMP_CMP_STR(1, SCMP_CMP_EQ, file));
   } else {
     return seccomp_rule_add_1(ctx, SCMP_ACT_ALLOW, SCMP_SYS(open),