tests/qtest/hd-geo-test: Fix checks on mkstemp() return value

Coverity notices that the checks against mkstemp() failing in
create_qcow2_with_mbr() are wrong: mkstemp returns -1 on failure but
the check is just "g_assert(fd)".  Fix to use "g_assert(fd >= 0)",
matching the correct check in create_test_img().

Fixes: Coverity CID 1432274
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-id: 20210525134458.6675-4-peter.maydell@linaro.org

tests/qtest/hd-geo-test.c

@@ -464,7 +464,7 @@ static char *create_qcow2_with_mbr(MBRpartitions mbr, uint64_t sectors)
     }
 
     fd = mkstemp(raw_path);
-    g_assert(fd);
+    g_assert(fd >= 0);
     close(fd);
 
     fd = open(raw_path, O_WRONLY);
@@ -474,7 +474,7 @@ static char *create_qcow2_with_mbr(MBRpartitions mbr, uint64_t sectors)
     close(fd);
 
     fd = mkstemp(qcow2_path);
-    g_assert(fd);
+    g_assert(fd >= 0);
     close(fd);
 
     qemu_img_path = getenv("QTEST_QEMU_IMG");