gas: missing (re-)initialization of local variable in fixup_segment()

gas: missing (re-)initialization of local variable in fixup_segment()

At the very least this has been causing bogus diagnostics, e.g.

	.text
	.data
	.long .bss - .
	.long -.text
	.bss

yielding

Error: can't resolve `0' {.bss section} - `.text' {.text section}

instead of

Error: can't resolve `0' {*ABS* section} - `.text' {.text section}

In particular for targets overriding any of TC_FORCE_RELOCATION_* & Co
or for ones setting md_register_arithmetic to true the problems may be
worse.