Segfault when invalid hostname is provided on connection.

<?php

$cb = new Couchbase("http://127.0.0.1:8091", ...); // the correct host should be just 127.0.0.1:8091

will result in a Segfault with the following backtrace:

#0 php_couchbase_error_callback (handle=0x7f861c056600, error=LIBCOUCHBASE_UNKNOWN_HOST, errinfo=0x7fff6de008d8 "Failed to look up \"http://127.0.0.1:8091\"") at couchbase.c:828
828 if (ctx->res->seqno < 0) {
(gdb) bt
#0 php_couchbase_error_callback (handle=0x7f861c056600, error=LIBCOUCHBASE_UNKNOWN_HOST, errinfo=0x7fff6de008d8 "Failed to look up \"http://127.0.0.1:8091\"") at couchbase.c:828
#1 0x000000010eefac1d in libcouchbase_error_handler ()
#2 0x000000010eefdbc8 in libcouchbase_connect ()
#3 0x000000010eeef1d5 in php_couchbase_create_impl () at couchbase.c:982
#4 0x000000010e5a5480 in zend_do_fcall_common_helper_SPEC ()
#5 0x000000010e5a5b5d in execute ()
#6 0x000000010e5416d9 in zend_execute_scripts ()
#7 0x000000010e4f203e in php_execute_script ()
#8 0x000000010e5bbcd8 in main ()

To resolve this we should:

a) not segfault but return a proper error
b) check if there is a http:// prefix in the supplied hostname, for convenience.