use after free in r_num_calc_index

[hannob]

Slightly different than #2855
https://crashes.fuzzing-project.org/radare2-script-oob-heap-read-r_num_calc_index-2

Content:
0
~[[0

Address Sanitizer:

==5680==ERROR: AddressSanitizer: heap-use-after-free on address 0x6020000156b3 at pc 0x7f8831e65d70 bp 0x7ffe1c067070 sp 0x7ffe1c067040
READ of size 2 at 0x6020000156b3 thread T0
    #0 0x7f8831e65d6f in strlen (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x32d6f)
    #1 0x7f882c529d1f in r_num_calc_index /f/radare2/radare2/libr/util/calc.c:152
    #2 0x7f88316e1d5f in num_callback /f/radare2/radare2/libr/core/core.c:149
    #3 0x7f882c480b26 in r_num_get /f/radare2/radare2/libr/util/num.c:118
    #4 0x7f8830f23a47 in r_cons_grep /f/radare2/radare2/libr/cons/grep.c:104
    #5 0x7f88317d9ea3 in r_core_cmd_subst_i /f/radare2/radare2/libr/core/cmd.c:1423
    #6 0x7f8831737dec in r_core_cmd_subst /f/radare2/radare2/libr/core/cmd.c:1084
    #7 0x7f8831739123 in r_core_cmd /f/radare2/radare2/libr/core/cmd.c:1947
    #8 0x7f883173bd3c in r_core_cmd_lines /f/radare2/radare2/libr/core/cmd.c:1998
    #9 0x7f883173c064 in r_core_cmd_file /f/radare2/radare2/libr/core/cmd.c:2010
    #10 0x7f883173f00e in r_core_run_script /f/radare2/radare2/libr/core/cmd.c:373
    #11 0x405416 in main /f/radare2/radare2/binr/radare2/radare2.c:729
    #12 0x7f882bc92f9f in __libc_start_main (/lib64/libc.so.6+0x1ff9f)
    #13 0x40a703 (/mnt/ram/r2/radare2+0x40a703)

0x6020000156b4 is located 0 bytes to the right of 4-byte region [0x6020000156b0,0x6020000156b4)
freed by thread T0 here:
    #0 0x7f8831e8a47f in __interceptor_free (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x5747f)
    #1 0x7f8831737ba7 in r_core_cmd_subst /f/radare2/radare2/libr/core/cmd.c:1108

previously allocated by thread T0 here:
    #0 0x7f8831e8a6f7 in malloc (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x576f7)
    #1 0x7f882bcf4789 in strdup (/lib64/libc.so.6+0x81789)

SUMMARY: AddressSanitizer: heap-use-after-free ??:0 strlen
Shadow bytes around the buggy address:
  0x0c047fffaa80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffaa90: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffaaa0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffaab0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffaac0: fa fa fa fa fa fa 02 fa fa fa 01 fa fa fa 05 fa
=>0x0c047fffaad0: fa fa fd fa fa fa[fd]fa fa fa fd fa fa fa fd fa
  0x0c047fffaae0: fa fa 07 fa fa fa 07 fa fa fa fd fd fa fa fd fd
  0x0c047fffaaf0: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fffab00: fa fa fd fa fa fa fd fa fa fa fd fd fa fa 02 fa
  0x0c047fffab10: fa fa fd fa fa fa fd fd fa fa fd fa fa fa fd fd
  0x0c047fffab20: fa fa fd fa fa fa fd fa fa fa fd fd fa fa fd fd
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
==5680==ABORTING

[radare]

just added a test case because the fix was exposing a regression