Fix return values for several syscall functions, patch by Zack Middleton (#5018)

code/client/cl_cgame.c

@@ -548,6 +548,7 @@ intptr_t CL_CgameSystemCalls( intptr_t *args ) {
 		return re.RegisterShaderNoMip( VMA(1) );
 	case CG_R_REGISTERFONT:
 		re.RegisterFont( VMA(1), args[2], VMA(3));
+		return 0;
 	case CG_R_CLEARSCENE:
 		re.ClearScene();
 		return 0;