Currently there is no measure in place to prevent the blob buffer
to exceed its maximum allowed length of 16MB. Continuously
calling blob_add() will expand the buffer until it exceeds
BLOB_ATTR_LEN_MASK and after that will return valid blob_attr
pointer without increasing the buflen.
A test program was added in the previous commit, this one fixes
the issue by asserting that the new bufflen after grow does not
Signed-off-by: Zefir Kurtisi <firstname.lastname@example.org>