Out of bounds call to strlen in r_bin_dwarf_parse_ext_opcode

[hannob]

An invalid elf file can cause an out of bounds memory access in the function r_bin_dwarf_parse_ext_opcode. Example file:
https://crashes.fuzzing-project.org/radare2-oob-heap-read-r_bin_dwarf_parse_ext_opcode

This was found with american fuzzy lop and can be seen with address sanitizer.

Address sanitizer output:

==29251==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60d00000ccb7 at pc 0x7f0bd3090d70 bp 0x7fff1864df00 sp 0x7fff1864ded0
READ of size 18 at 0x60d00000ccb7 thread T0
    #0 0x7f0bd3090d6f in strlen (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x32d6f)
    #1 0x7f0bd19f567d in r_bin_dwarf_parse_ext_opcode /f/radare2/radare2/libr/bin/dwarf.c:463
    #2 0x7f0bd19f567d in r_bin_dwarf_parse_opcodes /f/radare2/radare2/libr/bin/dwarf.c:666
    #3 0x7f0bd19f567d in r_bin_dwarf_parse_line_raw2 /f/radare2/radare2/libr/bin/dwarf.c:717
    #4 0x7f0bd19fe346 in r_bin_dwarf_parse_line /f/radare2/radare2/libr/bin/dwarf.c:1533
    #5 0x7f0bd2aa46e2 in bin_dwarf /f/radare2/radare2/libr/core/bin.c:472
    #6 0x7f0bd2aa46e2 in r_core_bin_info /f/radare2/radare2/libr/core/bin.c:1656
    #7 0x7f0bd2aa691d in r_core_bin_set_env /f/radare2/radare2/libr/core/bin.c:101
    #8 0x7f0bd2a3db3e in r_core_file_do_load_for_io_plugin /f/radare2/radare2/libr/core/file.c:355
    #9 0x7f0bd2a3db3e in r_core_bin_load /f/radare2/radare2/libr/core/file.c:487
    #10 0x405a3d in main /f/radare2/radare2/binr/radare2/radare2.c:585
    #11 0x7f0bccf28f9f in __libc_start_main (/lib64/libc.so.6+0x1ff9f)
    #12 0x40a0dd (/mnt/ram/radare2/radare2+0x40a0dd)

0x60d00000ccb7 is located 0 bytes to the right of 135-byte region [0x60d00000cc30,0x60d00000ccb7)
allocated by thread T0 here:
    #0 0x7f0bd30b5855 in calloc (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x57855)
    #1 0x7f0bd19fe277 in r_bin_dwarf_parse_line /f/radare2/radare2/libr/bin/dwarf.c:1525

SUMMARY: AddressSanitizer: heap-buffer-overflow ??:0 strlen
Shadow bytes around the buggy address:
  0x0c1a7fff9940: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff9950: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff9960: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff9970: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff9980: fa fa fa fa fa fa 00 00 00 00 00 00 00 00 00 00
=>0x0c1a7fff9990: 00 00 00 00 00 00[07]fa fa fa fa fa fa fa fa fa
  0x0c1a7fff99a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff99b0: 00 03 fa fa fa fa fa fa fa fa 00 00 00 00 00 00
  0x0c1a7fff99c0: 00 00 00 00 00 00 00 00 00 00 00 fa fa fa fa fa
  0x0c1a7fff99d0: fa fa fa fa 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff99e0: 00 00 00 00 00 fa fa fa fa fa fa fa fa fa 00 00
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
==29251==ABORTING