*Fix: Don't apply an aura when it's duration is 0. (for example because of diminishing returns)

This should fix interrupts when casting fear, stun, etc, while the duration is 0.
Fix based on havenard's patch.
Fixes issue 1621
Fixes issue 2271

--HG--
branch : trunk

src/game/Spell.cpp

@@ -1390,6 +1390,12 @@ SpellMissInfo Spell::DoSpellHitOnUnit(Unit *unit, const uint32 effectMask, bool
                 if (IsChanneledSpell(m_spellInfo))
                     m_originalCaster->ModSpellCastTime(aurSpellInfo, duration, this);
 
+                if (duration <= 0)
+                {
+                    m_spellAura->Remove();
+                    return SPELL_MISS_IMMUNE;
+                }
+
                 if (duration != m_spellAura->GetMaxDuration())
                 {
                     m_spellAura->SetMaxDuration(duration);