liblog: fix reading pmsg

d3ecc66b "liblog: support extended logger_entry headers" removed
the logger_entry::msg variable and instead uses hdr_size as an offset
from logger_entry to where the message starts in parent log_msg
buffer.

In pmsg, hdr_size is not recorded and therefore uninitialized when it
was referenced, causing corruption when reading last logcat.  This
change uses sizeof(log_msg->entry) instead.

Bug: 158263230
Test: last logcat works
Change-Id: Ic01e73bf4d8ba8419cc770138565aa1210a6078b