cmd/ld: fix off-by-one error when emitting symbol names

I diffed the output of `nm -n gofmt' before and after this change,
and verified that all changes are correct and all corrupted symbol
names are fixed.

Fixes #8906.

LGTM=iant, cookieo9
R=golang-codereviews, iant, cookieo9
CC=golang-codereviews
https://golang.org/cl/159750043

src/cmd/ld/macho.c

@@ -590,8 +590,7 @@ machosymtab(void)
 		if(strstr(s->extname, "·") == nil) {
 			addstring(symstr, s->extname);
 		} else {
-			p = s->extname;
-			while (*p++ != '\0') {
+			for(p = s->extname; *p; p++) {
 				if((uchar)*p == 0xc2 && (uchar)*(p+1) == 0xb7) {
 					adduint8(ctxt, symstr, '.');
 					p++;