Coverity notes that we don't check for dup2() failing.  Add some
assertions so that if it does ever happen we get some indication.
(This is similar to how we handle other "don't expect this syscall to
fail" checks in this test code.)

Fixes: Coverity CID 1432346
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-id: 20210525134458.6675-2-peter.maydell@linaro.org
---
 tests/qtest/bios-tables-test.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tests/qtest/bios-tables-test.c b/tests/qtest/bios-tables-test.c
index 156d4174aa3..51d3a4e2390 100644
--- a/tests/qtest/bios-tables-test.c
+++ b/tests/qtest/bios-tables-test.c
@@ -489,10 +489,14 @@ static void test_acpi_asl(test_data *data)
                                                  exp_sdt->asl_file, sdt->asl_file);
                     int out = dup(STDOUT_FILENO);
                     int ret G_GNUC_UNUSED;
+                    int dupret;
 
-                    dup2(STDERR_FILENO, STDOUT_FILENO);
+                    g_assert(out >= 0);
+                    dupret = dup2(STDERR_FILENO, STDOUT_FILENO);
+                    g_assert(dupret >= 0);
                     ret = system(diff) ;
-                    dup2(out, STDOUT_FILENO);
+                    dupret = dup2(out, STDOUT_FILENO);
+                    g_assert(dupret >= 0);
                     close(out);
                     g_free(diff);
                 }
-- 
2.20.1