FIX: Optimize fix_field_unparse() function

Micro-optimize fix_field_unparse() to save 2 μs on average:

  Before:

    Messages sent: 100000
    Round-trip time: min/avg/max = 22.0/22.9/451.0 μs
    Round-trip time: min/avg/max = 22.0/22.4/450.0 μs
    Round-trip time: min/avg/max = 22.0/22.6/448.0 μs
    Round-trip time: min/avg/max = 22.0/22.4/451.0 μs
    Round-trip time: min/avg/max = 22.0/22.5/443.0 μs
    Round-trip time: min/avg/max = 22.0/22.4/442.0 μs

  After:

    Messages sent: 100000
    Round-trip time: min/avg/max = 19.0/21.1/434.0 μs
    Round-trip time: min/avg/max = 19.0/20.3/444.0 μs
    Round-trip time: min/avg/max = 19.0/20.2/444.0 μs
    Round-trip time: min/avg/max = 19.0/20.3/443.0 μs
    Round-trip time: min/avg/max = 19.0/20.2/434.0 μs
    Round-trip time: min/avg/max = 19.0/20.2/434.0 μs

The vfprintf() function drops from 6% down to 3.86% in 'perf report':

  Before:

     6.80%  fix_client  [kernel.kallsyms]     [k] _raw_spin_lock_bh
     6.15%  fix_client  libc-2.17.so          [.] vfprintf
     3.43%  fix_client  libc-2.17.so          [.] __libc_recv
     3.30%  fix_client  [kernel.kallsyms]     [k] ipt_do_table
     3.15%  fix_client  [kernel.kallsyms]     [k] sock_recvmsg
     2.94%  fix_client  [kernel.kallsyms]     [k] tcp_recvmsg
     2.56%  fix_client  [kernel.kallsyms]     [k] __audit_syscall_exit
     2.56%  fix_client  [kernel.kallsyms]     [k] sock_has_perm
     2.39%  fix_client  [kernel.kallsyms]     [k] system_call
     2.14%  fix_client  [kernel.kallsyms]     [k] avc_has_perm_flags
     2.09%  fix_client  libc-2.17.so          [.] __printf_fp
     2.00%  fix_client  [kernel.kallsyms]     [k] SYSC_recvfrom
     1.97%  fix_client  [kernel.kallsyms]     [k] system_call_after_swapgs
     1.96%  fix_client  libc-2.17.so          [.] _IO_default_xsputn
     1.60%  fix_client  fix_client            [.] fix_message_parse

  After:

     6.19%  fix_client  [kernel.kallsyms]     [k] _raw_spin_lock_bh
     4.70%  fix_client  [kernel.kallsyms]     [k] ipt_do_table
     3.86%  fix_client  libc-2.17.so          [.] vfprintf
     3.49%  fix_client  libc-2.17.so          [.] __printf_fp
     2.93%  fix_client  fix_client            [.] fix_field_unparse
     2.40%  fix_client  libc-2.17.so          [.] __GI_____strtoll_l_internal
     2.37%  fix_client  [kernel.kallsyms]     [k] sock_recvmsg
     2.12%  fix_client  libc-2.17.so          [.] __libc_recv
     2.03%  fix_client  [kernel.kallsyms]     [k] tcp_recvmsg
     2.01%  fix_client  libc-2.17.so          [.] __GI_____strtod_l_internal
     1.99%  fix_client  libc-2.17.so          [.] __strftime_internal
     1.75%  fix_client  fix_client            [.] fix_message_parse

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