Use reference rather than copy in pt_get_protothread
Use reference rather than copy in pt_get_protothread

Older gcc's have difficulties optimizing this out into
a single dereference and it should have been a reference
in the first place (copies tend to get very expensive).
Paolo-Maffei/protothread,avr-uip/google-code-protothread,Paolo-Maffei/protothread,avr-uip/google-code-protothread,abhi-bit/protothread,abhi-bit/protothread