FIX: Micro-optimize parse_tag()

Before:

[penberg@localhost libtrading]$ perf stat taskset -c 2 tools/fix/fix_client -m 2 -h localhost -p 7070 -n 100000
Client Logon OK
Messages sent: 100000
Round-trip time: min/avg/max = 15.0/16.4/226.0 μs
Client Logout OK

 Performance counter stats for 'taskset -c 2 tools/fix/fix_client -m 2 -h localhost -p 7070 -n 100000':

       1693.787672      task-clock (msec)         #    0.998 CPUs utilized
                11      context-switches          #    0.006 K/sec
                 2      cpu-migrations            #    0.001 K/sec
               606      page-faults               #    0.358 K/sec
     5,342,018,512      cycles                    #    3.154 GHz                     [83.32%]
     2,478,820,731      stalled-cycles-frontend   #   46.40% frontend cycles idle    [83.32%]
     1,583,343,832      stalled-cycles-backend    #   29.64% backend  cycles idle    [66.75%]
     6,801,136,779      instructions              #    1.27  insns per cycle
                                                  #    0.36  stalled cycles per insn [83.38%]
     1,288,468,877      branches                  #  760.703 M/sec                   [83.37%]
         6,874,208      branch-misses             #    0.53% of all branches         [83.26%]

       1.696737850 seconds time elapsed

After:

[penberg@localhost libtrading]$ perf stat taskset -c 2 tools/fix/fix_client -m 2 -h localhost -p 7070 -n 100000
Client Logon OK
Messages sent: 100000
Round-trip time: min/avg/max = 15.0/15.4/433.0 μs
Client Logout OK

 Performance counter stats for 'taskset -c 2 tools/fix/fix_client -m 2 -h localhost -p 7070 -n 100000':

       1621.309108      task-clock (msec)         #    0.998 CPUs utilized
                10      context-switches          #    0.006 K/sec
                 5      cpu-migrations            #    0.003 K/sec
               607      page-faults               #    0.374 K/sec
     5,097,502,920      cycles                    #    3.144 GHz                     [83.31%]
     2,399,211,788      stalled-cycles-frontend   #   47.07% frontend cycles idle    [83.35%]
     1,528,311,447      stalled-cycles-backend    #   29.98% backend  cycles idle    [66.74%]
     6,353,038,642      instructions              #    1.25  insns per cycle
                                                  #    0.38  stalled cycles per insn [83.37%]
     1,172,638,013      branches                  #  723.266 M/sec                   [83.37%]
         6,673,377      branch-misses             #    0.57% of all branches         [83.25%]

       1.624034802 seconds time elapsed

Signed-off-by: Pekka Enberg <penberg@iki.fi>