tests: net: mqtt: Fix the error handling aftre read

If the rc = -EAGAIN from mqtt_read_publich_payload(), it shouldn't be
used in memcpy() since it is a negative value, and instead, it should
try to read again.

Fix: #13825
Coverity-CID: 191002

Signed-off-by: Tedd Ho-Jeong An <tedd.an@intel.com>
1 file changed