Attached input (only consisting of two bytes 00 ff) will result in an invalid memory read access. To see it you have to compile xmlif with address sanitizer (CFLAGS="-fsanitize=address").

Found with american fuzzy lop.

Address Sanitizer output:
{{{
==20701==ERROR: AddressSanitizer: global-buffer-overflow on address 0x00000040eb98 at pc 0x40a5fd bp 0x7fffbe27fb80 sp 0x7fffbe27fb70
READ of size 2 at 0x00000040eb98 thread T0
#0 0x40a5fc in yylex xmlif/xmlif.c:1417
#1 0x7f6ff4b6df9f in __libc_start_main (/lib64/libc.so.6+0x1ff9f)
#2 0x401961 (/mnt/ram/xmlto/xmlif/xmlif+0x401961)

0x00000040eb98 is located 0 bytes to the right of global variable 'yy_transition' from 'xmlif/xmlif.c' (0x40b540) of size 13912
SUMMARY: AddressSanitizer: global-buffer-overflow xmlif/xmlif.c:1417 yylex
Shadow bytes around the buggy address:
0x000080079d20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079d30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079d40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079d50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079d60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x000080079d70: 00 00 00[f9]f9 f9 f9 f9 00 00 00 00 00 00 00 00
0x000080079d80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079d90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079da0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079db0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000080079dc0: 00 00 00 00 00 00 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
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
==20701==ABORTING
}}}