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

target/i386/translate.c

@@ -4467,10 +4467,7 @@ static target_ulong disas_insn(DisasContext *s, CPUState *cpu)
     target_ulong pc_start = s->base.pc_next;
 
     s->pc_start = s->pc = pc_start;
-    prefixes = 0;
     s->override = -1;
-    rex_w = -1;
-    rex_r = 0;
 #ifdef TARGET_X86_64
     s->rex_x = 0;
     s->rex_b = 0;
@@ -4484,6 +4481,10 @@ static target_ulong disas_insn(DisasContext *s, CPUState *cpu)
         return s->pc;
     }
 
+    prefixes = 0;
+    rex_w = -1;
+    rex_r = 0;
+
  next_byte:
     b = x86_ldub_code(env, s);
     /* Collect prefixes.  */