Core/PathGenerator: Fix conditional jump on uninitialized value

Fix conditional jump on uninitialized value in PathGenerator::GetNavTerrain() by checking the result of getLiquidStatus() before accessing the LiquidData, uninitialized in case there's no water at all.

Valgrind log:
 Conditional jump or move depends on uninitialised value(s)
  at : PathGenerator::GetNavTerrain(float, float, float) (PathGenerator.cpp:553)
  by : PathGenerator::UpdateFilter() (PathGenerator.cpp:542)
  by : PathGenerator::CreateFilter() (PathGenerator.cpp:530)
  by : PathGenerator::PathGenerator(Unit const*) (PathGenerator.cpp:46)
  by : Spell::Spell(Unit*, SpellInfo const*, TriggerCastFlags, unsigned long, bool) (Spell.cpp:493)
  by : Unit::CastSpell(SpellCastTargets const&, SpellInfo const*, CustomSpellValues const*, TriggerCastFlags, Item*, AuraEffect const*, unsigned long) (Unit.cpp:846)
  by : Unit::CastSpell(Unit*, SpellInfo const*, TriggerCastFlags, Item*, AuraEffect const*, unsigned long) (Unit.cpp:882)
  by : Unit::CastSpell(Unit*, unsigned int, TriggerCastFlags, Item*, AuraEffect const*, unsigned long) (Unit.cpp:870)
  by : Unit::CastSpell(Unit*, unsigned int, bool, Item*, AuraEffect const*, unsigned long) (Unit.cpp:858)
  by : Player::SendInitialPacketsAfterAddToMap() (Player.cpp:22580)
  by : WorldSession::HandleMoveWorldportAckOpcode() (MovementHandler.cpp:121)
  by : WorldSession::HandleMoveWorldportAckOpcode(WorldPacket&) (MovementHandler.cpp:37)