mpd: update on 0.19.10 to fix build with gcc 5

0.19.1 failed with
<...>/src/db/plugins/simple/SimpleDatabasePlugin.cxx:448:10: error: converting to 'bool' from 'std::nullptr_t' requires direct-initialization [-fpermissive]
   return nullptr;
          ^
<...>/src/db/plugins/simple/SimpleDatabasePlugin.cxx:454:10: error: converting to 'bool' from 'std::nullptr_t' requires direct-initialization [-fpermissive]
   return nullptr;
          ^
<...>/src/db/plugins/simple/SimpleDatabasePlugin.cxx: In member function 'bool SimpleDatabase::Mount(const char*, const char*, Error&)':
<...>/src/db/plugins/simple/SimpleDatabasePlugin.cxx:481:10: error: converting to 'bool' from 'std::nullptr_t' requires direct-initialization [-fpermissive]
   return nullptr;
          ^

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>