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/library/pkcs7.c b/library/pkcs7.c
index 8c2a3ec..1c73709 100644
--- a/library/pkcs7.c
+++ b/library/pkcs7.c
@@ -523,8 +523,12 @@
         return( MBEDTLS_ERR_PKCS7_ALLOC_FAILED );
     }
 
-    mbedtls_md( md_info, data, datalen, hash );
-
+    ret = mbedtls_md( md_info, data, datalen, hash );
+    if( ret != 0 )
+    {
+        mbedtls_free( hash );
+        return( ret );
+    }
     ret = mbedtls_pk_verify( &pk_cxt, md_alg, hash, 0,
                                       pkcs7->signed_data.signers.sig.p,
                                       pkcs7->signed_data.signers.sig.len );