liblog: fix reading pmsg

d3ecc66b9c "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
Merged-In: Ic01e73bf4d8ba8419cc770138565aa1210a6078b
Change-Id: Ic01e73bf4d8ba8419cc770138565aa1210a6078b
(cherry picked from commit 34d7bd98adc080041202cceec4626fd88b4b6e88)

Former-commit-id: 709d56924170dc60dadbf3e69a27abe4c46c60ea

liblog/pmsg_reader.cpp

@@ -96,7 +96,7 @@ int PmsgRead(struct logger_list* logger_list, struct log_msg* log_msg) {
           ((logger_list->start.tv_sec != buf.l.realtime.tv_sec) ||
            (logger_list->start.tv_nsec <= buf.l.realtime.tv_nsec)))) &&
         (!logger_list->pid || (logger_list->pid == buf.p.pid))) {
-      char* msg = reinterpret_cast<char*>(&log_msg->entry) + log_msg->entry.hdr_size;
+      char* msg = reinterpret_cast<char*>(&log_msg->entry) + sizeof(log_msg->entry);
       *msg = buf.prio;
       fd = atomic_load(&logger_list->fd);
       if (fd <= 0) {