pkcs7.c: Do not ignore return value of mbedlts_md

CI was failing due to the return value of mbedtls_md being ignored.
If this function does fail, return early and propogate the md error.

Signed-off-by: Nick Child <nick.child@ibm.com>
diff --git a/tests/suites/test_suite_pkcs7.function b/tests/suites/test_suite_pkcs7.function
index d85a455..e2d76f3 100644
--- a/tests/suites/test_suite_pkcs7.function
+++ b/tests/suites/test_suite_pkcs7.function
@@ -285,9 +285,10 @@
 
     md_info = mbedtls_md_info_from_type( md_alg );
 
-    mbedtls_md( md_info, data, datalen, hash );
+    res = mbedtls_md( md_info, data, datalen, hash );
+    TEST_ASSERT( res == 0 );
 
-    res = mbedtls_pkcs7_signed_hash_verify( &pkcs7, &x509, hash, sizeof(hash));
+    res = mbedtls_pkcs7_signed_hash_verify( &pkcs7, &x509, hash, sizeof(hash) );
     TEST_ASSERT( res == 0 );
 
 exit: