Fix bug whereby 0 was written as 0200 rather than 020100

0200 is not just non-DER, it's completely invalid, since there has to be a
sign bit.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_asn1write.data b/tests/suites/test_suite_asn1write.data
index f2c32ec..6fc668f 100644
--- a/tests/suites/test_suite_asn1write.data
+++ b/tests/suites/test_suite_asn1write.data
@@ -91,8 +91,11 @@
 ASN.1 Write enum 2147483647
 mbedtls_asn1_write_enum:0x7fffffff:"0A047fffffff"
 
-#ASN.1 Write mpi 0
-#mbedtls_asn1_write_mpi:"00":"020100"
+ASN.1 Write mpi 0 (null)
+mbedtls_asn1_write_mpi:"":"020100"
+
+ASN.1 Write mpi 0 (1 limb)
+mbedtls_asn1_write_mpi:"00":"020100"
 
 ASN.1 Write mpi 1
 mbedtls_asn1_write_mpi:"01":"020101"