input: vcd: properly bail out on missing identifiers

input: vcd: properly bail out on missing identifiers

If we hit the missing identifier case, then we reached the end of the
token list. So, we should break out of the loop, and not continue.
Otherwise we will go past the end of the array as this minimal testcase
shows:

$timescale 1 ns $end
$var wire 1 n0 addr_0 $end
$enddefinitions $end
1

gives:

$ ./sigrok-cli -I vcd -i no_mod.vcd -O vcd -o /tmp/o.vcd
Segmentation fault

Signed-off-by: Wolfram Sang <redacted>