pkcs7: Ensure all data in asn1 structure is accounted for

Several PKCS7 invalid ASN1 Tests were failing due to extra
data bytes or incorrect content lengths going unnoticed. Make
the parser aware of possible malformed ASN1 data.

Signed-off-by: Nick Child <nick.child@ibm.com>
diff --git a/include/mbedtls/pkcs7.h b/include/mbedtls/pkcs7.h
index 835d2c1..b2cdabc 100644
--- a/include/mbedtls/pkcs7.h
+++ b/include/mbedtls/pkcs7.h
@@ -179,8 +179,9 @@
  * \brief          Parse a single DER formatted pkcs7 content.
  *
  * \param pkcs7    The pkcs7 structure to be filled by parser for the output.
- * \param buf      The buffer holding the DER encoded pkcs7.
- * \param buflen   The size in bytes of \p buf.
+ * \param buf      The buffer holding only the DER encoded pkcs7.
+ * \param buflen   The size in bytes of \p buf. The size must be exactly the
+ *                 length of the DER encoded pkcs7.
  *
  * \note           This function makes an internal copy of the PKCS7 buffer
  *                 \p buf. In particular, \p buf may be destroyed or reused