disas/i386.c: Add explicit braces round empty for-loop body

Add explicit braces round an empty for-loop body; this fits
QEMU style and is easier to read than an inconspicuous semicolon
at the end of the line. It also silences a clang warning:

disas/i386.c:4723:49: warning: for loop has empty body [-Wempty-body]
          for (i = 0; tmp[i] == '0' && tmp[i + 1]; i++);
                                                       ^
disas/i386.c:4723:49: note: put the semicolon on a separate line to silence this warning [-Wempty-body]

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
