matcher/phrasepostlist.cc,tests/api_backend.cc: If the anchor term
for a phrase is the first or last, set the min or max to its
position respectively.  Fixes the testcase in #657, but may not be
a complete fix.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18282 6b2f1b92-a4f3-0310-943f-de3cbe41748b