cfgfile: fix stack buffer underflow

cfgfile: fix stack buffer underflow

If cfgfile is give a line with comment character at the start
of the line, it will dereference outside of the buffer.

Detected with address sanitizer:

SUMMARY: AddressSanitizer: stack-buffer-underflow
lib/librte_cfgfile/rte_cfgfile.c:194 in rte_cfgfile_load_with_params
Shadow bytes around the buggy address:
  0x200fff79f6a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200fff79f6b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200fff79f6c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200fff79f6d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200fff79f6e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x200fff79f6f0: 00 00 00 00 f1 f1 f1[f1]00 00 00 00 00 00 00 00
  0x200fff79f700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200fff79f710: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x200fff79f720: 04 f2 f2 f2 f3 f3 f3 f3 00 00 00 00 00 00 00 00
  0x200fff79f730: 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 f2
  0x200fff79f740: f2 f2 f3 f3 f3 f3 00 00 00 00 00 00 00 00 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
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==2189==ABORTING

Fixes: a6a47ac9c2c9 ("cfgfile: rework load function")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>