of: overlay: validate offset from property fixups

The smatch static checker marks the data in offset as untrusted,
leading it to warn:

  drivers/of/resolver.c:125 update_usages_of_a_phandle_reference()
  error: buffer underflow 'prop->value' 's32min-s32max'

Add check to verify that offset is within the property data.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>