app/testpmd: fix hexadecimal parser with odd length

app/testpmd: fix hexadecimal parser with odd length

Current hex string parser assumes input has even characters number.
The parser fails input string with odd length.

The patch parses hex strings with even and odd length.
Parse result of an input with odd length will match result of
even length input, that has `0` as MSB, following by the original
sequence.
For example:
"0x1" results in *dst={0x01, 0x00}, *size=1
"0xabc" results in *dst={0x0a, 0xbc, 0x00}, *size=2

Fixes: 169a9fed1f4c ("app/testpmd: fix hex string parser support for flow API")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>