PR25197, assertion fail coffgen.c

PR25197, assertion fail coffgen.c

The testcase in this PR triggered "BFD_ASSERT (p2->is_sym)" by
sneakily generating a C_FILE sym whose value pointed into auxents.
The fix then is in the last changed line of this patch, to check
p->is_sym as well as p->u.syment.n_sclass.  The other changes fix
various overflow checks that weren't as solid as they could be.

	PR 25197
	* coffgen.c (coff_find_nearest_line_with_names): Check that C_FILE
	u.syment.n_value does point at another C_FILE sym and not into
	some auxent that happens to look like a C_FILE.  Properly check
	for integer overflow and avoid possible pointer wrap-around.
	Simplify pr17512 checks.