Always set the active source's update method

If the source is stopped, changes its buffer, then played again quickly, the
source will never be removed from the active source list causing the update
method to remain as it was. If the buffer was changed between mono and multi-
channel, this causes it to use the wrong method.

OpenAL32/alSource.c

@@ -2500,13 +2500,9 @@ ALvoid SetSourceState(ALsource *Source, ALCcontext *Context, ALenum state)
                 Context->ActiveSources[Context->ActiveSourceCount] = src;
             }
             memset(src, 0, sizeof(*src));
+            Context->ActiveSourceCount++;
 
             src->Source = Source;
-            if(BufferList->buffer->FmtChannels == FmtMono)
-                src->Update = CalcSourceParams;
-            else
-                src->Update = CalcNonAttnSourceParams;
-            Context->ActiveSourceCount++;
         }
         else
         {
@@ -2530,6 +2526,12 @@ ALvoid SetSourceState(ALsource *Source, ALCcontext *Context, ALenum state)
                 src->Send[i].Moving  = AL_FALSE;
             }
         }
+
+        if(BufferList->buffer->FmtChannels == FmtMono)
+            src->Update = CalcSourceParams;
+        else
+            src->Update = CalcNonAttnSourceParams;
+
         ATOMIC_STORE(&Source->NeedsUpdate, AL_TRUE);
     }
     else if(state == AL_PAUSED)