Clear *VAL in regcache_raw_read_unsigned

We have function regcache_raw_read_unsigned defined in both GDB and
GDBserver, so that it is used in common like this,

  ULONGEST value;
  status = regcache_raw_read_unsigned (regcache, regnum, &value);

'value' is correctly set in GDB side, but may not be correctly set
in GDBserver, because &value is passed in regcache_raw_read_unsigned
but collect_register may only set part of the whole variable.  In my
test, I see the top half of 'value' is garbage.  This patch fixes this
problem by clearing *VAL before calling collect_register.

gdb/gdbserver:

2016-02-10  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache_raw_read_unsigned): Clear *VAL.