PR22127, as segfaults assembling invalid .reloc

PR22127, as segfaults assembling invalid .reloc

"sec" gets set to NULL on errors in the offset expression.  This patch
disables part of the reloc expression processing that needs "sec"
valid.  I didn't disable the entire reloc expression handling so that
errors in the reloc expression are reported even when the offset
expression has an error.

	PR 22127
	* write.c (resolve_reloc_expr_symbols): Don't segfault when
	sec has been set to NULL.