Ingore SIGPIPE errors

If MTP exits unexpectedly, sending messages to add or remove a
storage device via Add_Remove_MTP_Storage was causing a crash.
Ignoring SIGPIPE allows us to more gracefully handle the error
instead of a total crash of TWRP.

Change-Id: I0a3f770f8b0f4bdbe10aa0ba857209b22b8668f7
Check for valid MTP_Storage_ID before adding or removing

Attempting to add a storage ID of 0 was causing a seg fault.

Change-Id: If8797186405be36ee70dbca63bd1063a62ba2812