Fix build breakage on GNU/Linux AArch64, take 2

Fix build breakage on GNU/Linux AArch64, take 2

The last commit unfortunately was not enough to fix the build breakage
on AArch64.  I made a mistake and did not test it alone on BuildBot,
but along with another patch that was responsible for fixing the
breakage.

The failure is:

  In file included from /usr/include/string.h:640:0,
		   from build-gnulib-gdbserver/import/string.h:41,
		   from ../../../binutils-gdb/gdb/gdbserver/../common/common-defs.h:56,
		   from ../../../binutils-gdb/gdb/gdbserver/server.h:22,
		   from ../../../binutils-gdb/gdb/gdbserver/regcache.c:19:
  In function ‘void* memset(void*, int, size_t)’,
      inlined from ‘regcache* init_register_cache(regcache*, const target_desc*, unsigned char*)’ at ../../../binutils-gdb/gdb/gdbserver/regcache.c:150:50:
  /usr/include/aarch64-linux-gnu/bits/string3.h:81:32: error: call to ‘__warn_memset_zero_len’ declared with attribute warning: memset used with constant zero length parameter; this could be due to transposed parameters [-Werror]
	 __warn_memset_zero_len ();
				  ^
  In function ‘void* memset(void*, int, size_t)’,
      inlined from ‘regcache* get_thread_regcache(thread_info*, int)’ at ../../../binutils-gdb/gdb/gdbserver/regcache.c:57:60:
  /usr/include/aarch64-linux-gnu/bits/string3.h:81:32: error: call to ‘__warn_memset_zero_len’ declared with attribute warning: memset used with constant zero length parameter; this could be due to transposed parameters [-Werror]
	 __warn_memset_zero_len ();

This is likely due to a GCC bug, because for some reason the compiler
assumes that the third argument to the memset:

  memset (regcache->register_status, REG_UNAVAILABLE,
	  VEC_length (tdesc_reg_p, regcache->tdesc->reg_defs));

is always zero, which is not always true.

Anyway, the simple fix for this is to guard the memset calls with:

  if (!VEC_empty (tdesc_reg_p, regcache->tdesc->reg_defs))

This time, I made sure to regtest only this patch on BuildBot, and it
finally solved the breakage.

gdb/gdbserver/ChangeLog:
2017-09-10  Sergio Durigan Junior  <sergiodj@redhat.com>

	* regcache.c (get_thread_regcache): Guard calls to "memset"
          with "!VEC_empty".