Is it safe to power a 3V 50mA device through a USB port?
No, but USB is 5.0V so if you add a suitable regulator you could power a 3V 50mA device. 

Inexpensive modules are available from China. You could even use a ~$1.50 USB->RS232 module based on chips such as CP2102, which have a 3.3V regulator on-board and ignore the serial port. 

If you directly apply 5V to a 3.3V device you run a high risk of permanently damaging it.