PR28402, fail to allocate line number array

This fixes a situation where the COFF code allocated memory for
internal representaion arrays before reading the external file data.
That meant the allocation didn't have any sanity check against file
size.

	PR 28402
	* coffcode.h (buy_and_read): Malloc rather than alloc memory.
	(coff_slurp_line_table): Read native line number info before
	allocating memory for internal line number array.  Adjust error
	paths to suit.  Remove now unnecessary line number count check.
	(coff_slurp_reloc_table): Adjust to suit buy_and_read change.