Backport change from trunk:
matcher/exactphrasepostlist.cc: We never try to build an
ExactPhrasePostList over 0 or 1 terms, so simply assert we don't,
rather than having an explicit check for it in the code.

git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18288 6b2f1b92-a4f3-0310-943f-de3cbe41748b