cmd/ld: don't allocate unused garbage space in pclntab file table

Fixes #6319.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13539043

src/cmd/ld/data.c

@@ -376,6 +376,12 @@ symgrow(Sym *s, int32 siz)
 	if(s->np >= siz)
 		return;
 
+	if(s->np > s->maxp) {
+		cursym = s;
+		diag("corrupt symbol data: np=%lld > maxp=%lld", (vlong)s->np, (vlong)s->maxp);
+		errorexit();
+	}
+
 	if(s->maxp < siz) {
 		if(s->maxp == 0)
 			s->maxp = 8;

src/cmd/ld/lib.c

@@ -2348,7 +2348,6 @@ ftabaddstring(Sym *ftab, char *s)
 	start = ftab->np;
 	symgrow(ftab, start+n+1);
 	strcpy((char*)ftab->p + start, s);
-	ftab->np += n+1;
 	return start;
 }