tests/qtest/bios-tables-test: Check for dup2() failure

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

@@ -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);
                 }