target/i386: Fix compiler warnings

These gcc warnings are fixed:

target/i386/translate.c:4461:12: warning:
 variable 'prefixes' might be clobbered by 'longjmp' or 'vfork' [-Wclobbered]
target/i386/translate.c:4466:9: warning:
 variable 'rex_w' might be clobbered by 'longjmp' or 'vfork' [-Wclobbered]
target/i386/translate.c:4466:16: warning:
 variable 'rex_r' might be clobbered by 'longjmp' or 'vfork' [-Wclobbered]

Tested with x86_64-w64-mingw32-gcc from Debian stretch.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20171113064845.29142-1-sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>