fix keyvalues encode crash

Accomplished in exchange for my sanity, and a peek at Valve's work.

Effectively, the bug is that m_pSub can be null, so when it does
`dat->m_pSub->WriteAsBinary( buffer );`, it would be running
WriteAsBinary with `this` set to null. So, if we detect a null m_pSub,
we write a empty keyvalues structure manually by writing the ending
marker directly. The previous "fix" for this accidentally did exactly
this, just in a more roundabout way.

Valve's solution seems to go about this fix by checking if `this` is
null, at least in the decompiled code, but that method didn't work for
me (presumably due to compiler optimizations).

Author: ReplayCoding

src/tier1/KeyValues.cpp

@@ -2607,7 +2607,15 @@ bool KeyValues::WriteAsBinary( CUtlBuffer &buffer )
 		{
 		case TYPE_NONE:
 			{
-				dat->m_pSub->WriteAsBinary( buffer );
+				if (dat->m_pSub)
+				{
+					dat->m_pSub->WriteAsBinary( buffer );
+				}
+				else {
+					// There was no m_pSub, create a fake empty KeyValues manually.
+					// write tail, marks end of peers
+					buffer.PutUnsignedChar( TYPE_NUMTYPES ); 
+				}
 				break;
 			}
 		case TYPE_STRING: