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.