Fix "set architecture mips:10000" crash

Fix "set architecture mips:10000" crash

Fix this GDB crash:

  $ gdb -ex "set architecture mips:10000"
  Segmentation fault (core dumped)

Backtrace:

  Program received signal SIGSEGV, Segmentation fault.
  0x0000000000495b1b in mips_gdbarch_init (info=..., arches=0x0) at /home/pedro/gdb/mygit/cxx-convertion/src/gdb/mips-tdep.c:8436
  8436              if (bfd_get_flavour (info.abfd) == bfd_target_elf_flavour
  (top-gdb) bt
  #0  0x0000000000495b1b in mips_gdbarch_init (info=..., arches=0x0) at .../src/gdb/mips-tdep.c:8436
  #1  0x00000000007348a6 in gdbarch_find_by_info (info=...) at .../src/gdb/gdbarch.c:5155
  #2  0x000000000073563c in gdbarch_update_p (info=...) at .../src/gdb/arch-utils.c:522
  #3  0x0000000000735585 in set_architecture (ignore_args=0x0, from_tty=1, c=0x26bc870) at .../src/gdb/arch-utils.c:496
  #4  0x00000000005f29fd in do_sfunc (c=0x26bc870, args=0x0, from_tty=1) at .../src/gdb/cli/cli-decode.c:121
  #5  0x00000000005fd3f3 in do_set_command (arg=0x7fffffffdcdd "mips:10000", from_tty=1, c=0x26bc870) at .../src/gdb/cli/cli-setshow.c:455
  #6  0x0000000000836157 in execute_command (p=0x7fffffffdcdd "mips:10000", from_tty=1) at .../src/gdb/top.c:460
  #7  0x000000000071abfb in catch_command_errors (command=0x835f6b <execute_command>, arg=0x7fffffffdccc "set architecture mips:10000", from_tty=1)
      at .../src/gdb/main.c:368
  #8  0x000000000071bf4f in captured_main (data=0x7fffffffd750) at .../src/gdb/main.c:1132
  #9  0x0000000000716737 in catch_errors (func=0x71af44 <captured_main>, func_args=0x7fffffffd750, errstring=0x106b9a1 "", mask=RETURN_MASK_ALL)
      at .../src/gdb/exceptions.c:240
  #10 0x000000000071bfe6 in gdb_main (args=0x7fffffffd750) at .../src/gdb/main.c:1164
  #11 0x000000000040a6ad in main (argc=4, argv=0x7fffffffd858) at .../src/gdb/gdb.c:32
  (top-gdb)

We already check whether info.abfd is NULL before all other
bfd_get_flavour calls in the same function.  Just this one case was
missing.

(This was exposed by a WIP test that tries all "set architecture ARCH"
values.)

gdb/ChangeLog:
2016-03-07  Pedro Alves  <palves@redhat.com>

	* mips-tdep.c (mips_gdbarch_init): Check whether info.abfd is NULL
	before calling bfd_get_flavour.