Minor RageSound improvements

Nothing major here - updating C style casts, moving repeated function calls into variables, changing lrint's...

**RageSound.cpp**

- Initialize m_pSource to nullptr in the member initializer list, rather than in the body of the constructor
- Define an undefined variable `iSourceFrame`
- Change a `lrint` to a `static_cast<int>+0.5`
- Implement missing error handling in `SetStopModeFromString` with a log message

**RageSoundManager**

- Combined the iterator increment and the erase operation for `Update` into one line to prevent needing to create the `next` variable, since GameLoop calls this method frequently

**RageSoundReader**

- This method is called from RageSound just after making sure iFrames isn't equal to 0, so it's not needed for RageSoundReader to do it again.
- We will never fail to read a file 100 times. If we do, it's because of I/O errors, so make that more clear.

src/RageSoundManager.cpp

@@ -112,20 +112,18 @@ void RageSoundManager::Update()
 	/* Scan m_mapPreloadedSounds for sounds that are no longer loaded, and delete them. */
 	g_SoundManMutex.Lock(); /* lock for access to m_mapPreloadedSounds, owned_sounds */
 	{
-		std::map<RString, RageSoundReader_Preload*>::iterator it, next;
-		it = m_mapPreloadedSounds.begin();
-
-		while( it != m_mapPreloadedSounds.end() )
+		for( auto it = m_mapPreloadedSounds.begin(); it != m_mapPreloadedSounds.end(); )
 		{
-			next = it; ++next;
 			if( it->second->GetReferenceCount() == 1 )
 			{
 				LOG->Trace( "Deleted old sound \"%s\"", it->first.c_str() );
 				delete it->second;
-				m_mapPreloadedSounds.erase( it );
+				it = m_mapPreloadedSounds.erase(it);
+			}
+			else
+			{
+				++it;
 			}
-
-			it = next;
 		}
 	}