jshn: fix off by one in jshn_parse_file

jshn: fix off by one in jshn_parse_file

Fixes following error:

 Invalid read of size 1
   at 0x4C32D04: strlen
   by 0x5043367: json_tokener_parse_ex
   by 0x5045316: json_tokener_parse_verbose
   by 0x504537D: json_tokener_parse
   by 0x401AB1: jshn_parse (jshn.c:179)
   by 0x40190D: jshn_parse_file (jshn.c:370)
   by 0x40190D: main (jshn.c:434)
 Address 0x5848c4c is 0 bytes after a block of size 1,036 alloc'd
   at 0x4C2FB0F: malloc
   by 0x4018E2: jshn_parse_file (jshn.c:357)
   by 0x4018E2: main (jshn.c:434)

Signed-off-by: Petr Štetiar <ynezz@true.cz>