Also, if the header is bad, exit with a non-zero status.

Other calls to Brdline in the tree, by category:

Reading symbol name from object file:
./cmd/5l/obj.c:486: 		name = Brdline(f, '\0');
./cmd/6l/obj.c:535: 		name = Brdline(f, '\0');
./cmd/8l/obj.c:564: 		name = Brdline(f, '\0');
./libmach/sym.c:292: 		cp = Brdline(bp, '\0');

Reading archive header line (fixed, short):
./cmd/gc/lex.c:287: 	if((a = Brdline(b, '\n')) == nil)
./cmd/gc/lex.c:303: 	if((p = Brdline(b, '\n')) == nil)

Reading object file header line (fixed, short):
./cmd/ld/lib.c:421: 	line = Brdline(f, '\n');

Reading undefined symbol list (unused code):
./cmd/ld/lib.c:773: 	while((l = Brdline(b, '\n')) != nil){

Implementing Brdstr:
./libbio/brdstr.c:36: 		p = Brdline(bp, delim);

The symbol names ones will cause a problem loudly if they
fail: they'll error out with symbol name too long.  This means
that you can't define an enormous struct without giving the
type a name and then stick it in an interface, because the
type's symbol name will be too long for the object file.
Since this will be a loud failure instead of a silent one,
I'm willing to wait until it comes up in practice.

R=r
CC=golang-dev
https://golang.org/cl/1982041