Call to erase() invalidates iterators so switch from range-based for
loop to using iterators manually.

This fixes a bug that resulted in JVM crashing on SMP when "perf
callstack" was run:

  #
  # A fatal error has been detected by the Java Runtime Environment:
  #
  #  SIGSEGV (0xb) at pc=0x0000000000328a44, pid=0, tid=18446673706080178176
  #
  # JRE version: 7.0_19
  # Java VM: OpenJDK 64-Bit Server VM (23.7-b01 mixed mode linux-amd64 compressed oops)
  # Problematic frame:
  # C  0x0000000000328a44
  #
  # Core dump written. Default location: //core or core.0
  #
  # An error report file with more information is saved as:
  # /tmp/jvm-0/hs_error.log
  #
  # If you would like to submit a bug report, please include
  # instructions on how to reproduce the bug and visit:
  #   http://icedtea.classpath.org/bugzilla
  #
  Aborted