out of bounds heap read in cmd_flag

[hannob]

This script causes an out of bounds heap read:
https://crashes.fuzzing-project.org/radare2-script-oob-heap-read-cmd_flag

It contains the string "fR". Test: radare2 -q -i [script] /dev/null
Found with american fuzzy lop.

Address Sanitizer trace:

==7159==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000016871 at pc 0x7f35c21507d0 bp 0x7fff1db3d3c0 sp 0x7fff1db3d390
READ of size 1 at 0x602000016871 thread T0
    #0 0x7f35c21507cf in index (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x347cf)
    #1 0x7f35c1aa4290 in cmd_flag /f/radare2/radare2/libr/core/cmd_flag.c:120
    #2 0x7f35c1ae5096 in r_core_cmd_subst_i /f/radare2/radare2/libr/core/cmd.c:1590
    #3 0x7f35c1a4909c in r_core_cmd_subst /f/radare2/radare2/libr/core/cmd.c:1081
    #4 0x7f35c1a4a182 in r_core_cmd /f/radare2/radare2/libr/core/cmd.c:1938
    #5 0x7f35c1a4d084 in r_core_cmd_lines /f/radare2/radare2/libr/core/cmd.c:1989
    #6 0x7f35c1a4d2f4 in r_core_cmd_file /f/radare2/radare2/libr/core/cmd.c:2001
    #7 0x7f35c1a5028b in r_core_run_script /f/radare2/radare2/libr/core/cmd.c:373
    #8 0x4054bc in main /f/radare2/radare2/binr/radare2/radare2.c:729
    #9 0x7f35bbfb3f9f in __libc_start_main (/lib64/libc.so.6+0x1ff9f)
    #10 0x40a4d3 (/mnt/ram/radare2/radare2+0x40a4d3)

0x602000016871 is located 0 bytes to the right of 1-byte region [0x602000016870,0x602000016871)
allocated by thread T0 here:
    #0 0x7f35c21736f7 in malloc (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x576f7)
    #1 0x7f35bc015789 in strdup (/lib64/libc.so.6+0x81789)

SUMMARY: AddressSanitizer: heap-buffer-overflow ??:0 index
Shadow bytes around the buggy address:
  0x0c047fffacb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffacc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffacd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047ffface0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffacf0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c047fffad00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa[01]fa
  0x0c047fffad10: fa fa 01 fa fa fa 03 fa fa fa 03 fa fa fa 03 fa
  0x0c047fffad20: fa fa fd fa fa fa fd fd fa fa 06 fa fa fa fd fa
  0x0c047fffad30: fa fa 05 fa fa fa fd fa fa fa 06 fa fa fa fd fa
  0x0c047fffad40: fa fa 03 fa fa fa fd fa fa fa 04 fa fa fa fd fa
  0x0c047fffad50: fa fa fd fa fa fa 04 fa fa fa fd fa fa fa fd fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Contiguous container OOB:fc
  ASan internal:           fe
==7159==ABORTING