Move "fastboot oem" to std::string.

Not sure how this code got missed when we moved everything else off
C string handling...

  $ adb reboot bootloader
  $ fastboot oem `perl -e 'print "x"x1024;'`

Before:
  <crashes>

After:
  error: Command length (1028) exceeds maximum size (64)

(The error says 1028 instead of 1024 because it includes the "oem ".)

Bug: http://b/36232671
Test: fastboot oem `perl -e 'print "x"x1024;'`
Change-Id: Ib4664e49222bd2b71be5aa3fe81f386d6073414f