gdbstub: i386: Fix gdb_get_reg16() parameter to unbreak gdb

We should only pass in gdb_get_reg16() with the GByteArray* object
itself, no need to shift.  Without this patch, gdb remote attach will
crash QEMU:

  (gdb) target remote :1234
  Remote debugging using :1234
  Remote communication error.  Target disconnected.: Connection reset by peer.
  $ qemu-system-x86_64 -m 1G -smp 4 ... -s
  ERROR:qemu/gdbstub.c:1843:handle_read_all_regs: assertion failed: (len == gdbserver_state.mem_buf->len)
  Bail out! ERROR:qemu/gdbstub.c:1843:handle_read_all_regs: assertion failed: (len == gdbserver_state.mem_buf->len)

Fixes: a010bdb ("extend GByteArray to read register helpers")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200409164954.36902-3-peterx@redhat.com>
Message-Id: <20200414200631.12799-12-alex.bennee@linaro.org>

target/i386/gdbstub.c

@@ -106,7 +106,7 @@ int x86_cpu_gdb_read_register(CPUState *cs, GByteArray *mem_buf, int n)
     } else if (n >= IDX_FP_REGS && n < IDX_FP_REGS + 8) {
         floatx80 *fp = (floatx80 *) &env->fpregs[n - IDX_FP_REGS];
         int len = gdb_get_reg64(mem_buf, cpu_to_le64(fp->low));
-        len += gdb_get_reg16(mem_buf + len, cpu_to_le16(fp->high));
+        len += gdb_get_reg16(mem_buf, cpu_to_le16(fp->high));
         return len;
     } else if (n >= IDX_XMM_REGS && n < IDX_XMM_REGS + CPU_NB_REGS) {
         n -= IDX_XMM_REGS;