Fix memcpy() UB in mbedtls_asn1_named_data()

Removes a case in mbedtls_asn1_named_data() where memcpy() could be
called with a null pointer and zero length. A test case is added for
this code path, to catch the undefined behavior when running tests with
UBSan.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/tests/suites/test_suite_asn1write.data b/tests/suites/test_suite_asn1write.data
index 7f5f536..f844d48 100644
--- a/tests/suites/test_suite_asn1write.data
+++ b/tests/suites/test_suite_asn1write.data
@@ -374,10 +374,13 @@
 store_named_data_val_found:4:9
 
 Store named data: new, val_len=0
-store_named_data_val_new:0
+store_named_data_val_new:0:1
+
+Stored named data: new, val_len=0, val=NULL
+store_named_data_val_new:0:0
 
 Store named data: new, val_len=4
-store_named_data_val_new:4
+store_named_data_val_new:4:1
 
 Store named data: new, val_len=4, val=NULL
-store_named_data_val_new:-4
+store_named_data_val_new:4:0