re PR go/61303 (gccgo: segfault, regression since 4.8.2)

PR go/61303
    runtime: don't overallocate in select code
    
    If we've already allocated an fd_set, don't allocate another one.
    
    Also, don't bother to read from rdwake if it wasn't returned in select.
    
    Fixes https://gcc.gnu.org/PR61303.
    
    Reviewed-on: https://go-review.googlesource.com/17243

From-SVN: r230923

libgo/runtime/netpoll_select.c

@@ -135,6 +135,8 @@ runtime_netpoll(bool block)
 	byte b;
 	struct stat st;
 
+	allocatedfds = false;
+
  retry:
 	runtime_lock(&selectlock);
 
@@ -146,11 +148,13 @@ runtime_netpoll(bool block)
 	}
 
 	if(inuse) {
+		if(!allocatedfds) {
 			prfds = runtime_SysAlloc(4 * sizeof fds, &mstats.other_sys);
 			pwfds = prfds + 1;
 			pefds = pwfds + 1;
 			ptfds = pefds + 1;
 			allocatedfds = true;
+		}
 	} else {
 		prfds = &grfds;
 		pwfds = &gwfds;
@@ -216,7 +220,7 @@ runtime_netpoll(bool block)
 			mode = 'r' + 'w';
 			--c;
 		}
-		if(i == rdwake) {
+		if(i == rdwake && mode != 0) {
 			while(read(rdwake, &b, sizeof b) > 0)
 				;
 			continue;