detect: fix buffer length to uint32

detect: fix buffer length to uint32

There is a difference in the size of the buffer length as passed from
the content buffers (cfr HttpReassembledBody.buffer_len) and the buflen
variable passed to mpm primitives. This can cause a misdetection
whenever the bufferlen is multiple of 65536 (as uint16(X*65536) == 0).
Increasing the buflen variable type to uint32 solves the issue (this
does not cause any issue with primitives, they all accept uint32).