BUG/MINOR: errors: handle malloc failure in usermsgs_put()

BUG/MINOR: errors: handle malloc failure in usermsgs_put()

usermsgs_buf.size is set without first checking if previous malloc
attempt succeeded.

This could fool the buffer API into assuming that the buffer is
initialized, resulting in unsafe read/writes.

Guarding usermsgs_buf.size assignment with the malloc attempt result
to make the buffer initialization safe against malloc failures.

This partially fixes GH #2130.

It should be backported up to 2.6.