check for overflow vertex indices, fixes issue 6135

thx @ JMC47 for identifying the reversion, creating a useful bug report with fifo log :-)

Source/Core/VideoCommon/Src/IndexGenerator.cpp

@@ -166,3 +166,10 @@ void IndexGenerator::AddPoints(u32 numVerts)
 		++numP;
 	}
 }
+
+
+u32 IndexGenerator::GetRemainingIndices()
+{
+	u32 max_index = 65535;
+	return max_index - index;
+}

Source/Core/VideoCommon/Src/IndexGenerator.h

@@ -41,6 +41,8 @@ class IndexGenerator
 	static u32 GetTriangleindexLen() {return (u32)(Tptr - BASETptr);}
 	static u32 GetLineindexLen() {return (u32)(Lptr - BASELptr);}
 	static u32 GetPointindexLen() {return (u32)(Pptr - BASEPptr);}
+
+	static u32 GetRemainingIndices();
 /*
 	enum IndexPrimitiveType
 	{

Source/Core/VideoCommon/Src/VertexManagerBase.cpp

@@ -52,16 +52,18 @@ void VertexManager::PrepareForAdditionalData(int primitive, u32 count, u32 strid
 {	
 	u32 const needed_vertex_bytes = count * stride;
 
-	if (needed_vertex_bytes > GetRemainingSize() || count > GetRemainingIndices(primitive))
+	if (count > IndexGenerator::GetRemainingIndices() || count > GetRemainingIndices(primitive) || needed_vertex_bytes > GetRemainingSize())
 	{
 		Flush();
 
-		if (needed_vertex_bytes > GetRemainingSize())
-			ERROR_LOG(VIDEO, "VertexManager: Buffer not large enough for all vertices! "
-				"Increase MAXVBUFFERSIZE or we need primitive breaking afterall.");
+		if(count > IndexGenerator::GetRemainingIndices())
+			ERROR_LOG(VIDEO, "Too less index values. Use 32bit or reset them on flush.");
 		if (count > GetRemainingIndices(primitive))
 			ERROR_LOG(VIDEO, "VertexManager: Buffer not large enough for all indices! "
 				"Increase MAXIBUFFERSIZE or we need primitive breaking afterall.");
+		if (needed_vertex_bytes > GetRemainingSize())
+			ERROR_LOG(VIDEO, "VertexManager: Buffer not large enough for all vertices! "
+				"Increase MAXVBUFFERSIZE or we need primitive breaking afterall.");
 	}
 }