qcommon: Fix VMs being able to change CVAR_PROTECTED cvars

VM could use Cvar_Register to set a protected cvar as user created and
was then able to use Cvar_Register with CVAR_ROM to change the value.

Don't allow Cvar_Register to affect protected cvars and prevent VMs
from adding internal flags to any cvars (creator, modified, protected,
nonexistent).

Reported by Noah Metzger (Chomenor).

src/qcommon/cvar.c

@@ -1693,7 +1693,46 @@ void Cvar_Register(vmCvar_t *vmCvar, const char *varName, const char *defaultVal
 		return;
 	}
 
-	cv = Cvar_Get(varName, defaultValue, flags | CVAR_VM_CREATED);
+	// Don't allow VM to specific a different creator or other internal flags.
+	if (flags & CVAR_USER_CREATED)
+	{
+		Com_Printf(S_COLOR_YELLOW "WARNING: VM tried to set CVAR_USER_CREATED on cvar '%s'\n", varName);
+		flags &= ~CVAR_USER_CREATED;
+	}
+	if (flags & CVAR_SERVER_CREATED)
+	{
+		Com_Printf(S_COLOR_YELLOW "WARNING: VM tried to set CVAR_SERVER_CREATED on cvar '%s'\n", varName);
+		flags &= ~CVAR_SERVER_CREATED;
+	}
+	if (flags & CVAR_PROTECTED)
+	{
+		Com_Printf(S_COLOR_YELLOW "WARNING: VM tried to set CVAR_PROTECTED on cvar '%s'\n", varName);
+		flags &= ~CVAR_PROTECTED;
+	}
+	if (flags & CVAR_MODIFIED)
+	{
+		Com_Printf(S_COLOR_YELLOW "WARNING: VM tried to set CVAR_MODIFIED on cvar '%s'\n", varName);
+		flags &= ~CVAR_MODIFIED;
+	}
+	if (flags & CVAR_NONEXISTENT)
+	{
+		Com_Printf(S_COLOR_YELLOW "WARNING: VM tried to set CVAR_NONEXISTENT on cvar '%s'\n", varName);
+		flags &= ~CVAR_NONEXISTENT;
+	}
+
+	cv = Cvar_FindVar(varName);
+
+	// Don't modify cvar if it's protected.
+	if (cv && (cv->flags & CVAR_PROTECTED))
+	{
+		Com_Printf(S_COLOR_YELLOW "WARNING: VM tried to register protected cvar '%s' with value '%s'%s\n",
+		varName, defaultValue, (flags & ~cv->flags ) != 0 ? " and new flags" : "" );
+	}
+	else
+	{
+		cv = Cvar_Get(varName, defaultValue, flags | CVAR_VM_CREATED);
+	}
+
 	if (!vmCvar)
 	{
 		return;