net/mqtt: Validate null or app level zero-length rx buffers

Don't try to process null or zero-length buffers generated by
the IP stack. Zero-length buffers are valid at the TCP layer but
contain no information for applications.

Change-Id: If66d301527f56ca8e8761789b7fd6931fc37b8e0
Signed-off-by: Flavio Santes <flavio.santes@intel.com>
1 file changed