Fix mincore() to deal with unmapped addresses like msync() does.

This fixes a SIGSEGV in libunwind's access_mem() when leak detector is
enabled:

   (gdb) bt
  #0  page_fault (ef=0xffffc0003ffe7008) at ../../core/mmu.cc:871
  #1  <signal handler called>
  #2  ContiguousSpace::block_start_const (this=<optimized out>, p=0x77d2f3968)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/share/vm/oops/oop.inline.hpp:411
  #3  0x00001000008ae16c in GenerationBlockStartClosure::do_space (this=0x2000001f9100, s=<optimized out>)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/share/vm/memory/generation.cpp:242
  #4  0x00001000007f097c in DefNewGeneration::space_iterate (this=0xffffc0003fb68c00, blk=0x2000001f9100, usedOnly=<optimized out>)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/share/vm/memory/defNewGeneration.cpp:480
  #5  0x00001000008aca0e in Generation::block_start (this=<optimized out>, p=<optimized out>)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/share/vm/memory/generation.cpp:251
  #6  0x0000100000b06d2f in os::print_location (st=st@entry=0x2000001f9560, x=32165017960, verbose=verbose@entry=false)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/share/vm/runtime/os.cpp:868
  #7  0x0000100000b11b5b in os::print_register_info (st=0x2000001f9560, context=0x2000001f9740)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/os_cpu/linux_x86/vm/os_linux_x86.cpp:839
  #8  0x0000100000c6cde8 in VMError::report (this=0x2000001f9610, st=st@entry=0x2000001f9560)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/share/vm/utilities/vmError.cpp:551
  #9  0x0000100000c6da3b in VMError::report_and_die (this=this@entry=0x2000001f9610)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/share/vm/utilities/vmError.cpp:984
  #10 0x0000100000b1109f in JVM_handle_linux_signal (sig=11, info=0x2000001f9bb8, ucVoid=0x2000001f9740,
      abort_if_unrecognized=<optimized out>)
      at /usr/src/debug/java-1.7.0-openjdk-1.7.0.25-2.3.12.3.fc19.x86_64/openjdk/hotspot/src/os_cpu/linux_x86/vm/os_linux_x86.cpp:528
  #11 0x000000000039f242 in call_signal_handler (frame=0x2000001f9b10) at ../../arch/x64/signal.cc:69
  #12 <signal handler called>
  #13 0x000000000057d721 in access_mem ()
  #14 0x000000000057cb1d in dwarf_get ()
  #15 0x000000000057ce51 in _ULx86_64_step ()
  #16 0x00000000004315fd in backtrace (buffer=0x1ff9d80 <memory::alloc_tracker::remember(void*, int)::bt>, size=20)
      at ../../libc/misc/backtrace.cc:16
  #17 0x00000000003b8d99 in memory::alloc_tracker::remember (this=0x1777ae0 <memory::tracker>, addr=0xffffc0004508de00, size=54)
      at ../../core/alloctracker.cc:59
  #18 0x00000000003b0504 in memory::tracker_remember (addr=0xffffc0004508de00, size=54) at ../../core/mempool.cc:43
  #19 0x00000000003b2152 in std_malloc (size=54) at ../../core/mempool.cc:723
  #20 0x00000000003b259c in malloc (size=54) at ../../core/mempool.cc:856
  #21 0x0000100001615e4c in JNU_GetStringPlatformChars (env=env@entry=0xffffc0003a4dc1d8, jstr=jstr@entry=0xffffc0004591b800,
      isCopy=isCopy@entry=0x0) at ../../../src/share/native/common/jni_util.c:801
  #22 0x000010000161ada6 in Java_java_io_UnixFileSystem_getBooleanAttributes0 (env=0xffffc0003a4dc1d8, this=<optimized out>,
      file=<optimized out>) at ../../../src/solaris/native/java/io/UnixFileSystem_md.c:111
  #23 0x000020000021ed8e in ?? ()
  #24 0x00002000001faa58 in ?? ()
  #25 0x00002000001faac0 in ?? ()
  #26 0x00002000001faa50 in ?? ()
  #27 0x0000000000000000 in ?? ()

Spotted by Avi Kivity.