runtime: more graceful out-of-memory crash

Used to fault trying to access l->list->next
when l->list == nil after MCentral_AllocList.
Now prints

runtime: out of memory: no room in arena for 65536-byte allocation (536870912 in use)
throw: out of memory

followed by stack trace.

Fixes #1650.

R=r, dfc
CC=golang-dev
https://golang.org/cl/4446062

src/pkg/runtime/mcache.c

@@ -22,6 +22,8 @@ runtime·MCache_Alloc(MCache *c, int32 sizeclass, uintptr size, int32 zeroed)
 		// Replenish using central lists.
 		n = runtime·MCentral_AllocList(&runtime·mheap.central[sizeclass],
 			runtime·class_to_transfercount[sizeclass], &first);
+		if(n == 0)
+			runtime·throw("out of memory");
 		l->list = first;
 		l->nlist = n;
 		c->size += n*size;

src/pkg/runtime/mheap.c

@@ -180,8 +180,10 @@ MHeap_Grow(MHeap *h, uintptr npage)
 	// Allocate a multiple of 64kB (16 pages).
 	npage = (npage+15)&~15;
 	ask = npage<<PageShift;
-	if(ask > h->arena_end - h->arena_used)
+	if(ask > h->arena_end - h->arena_used) {
+		runtime·printf("runtime: out of memory: no room in arena for %D-byte allocation (%D in use)\n", (uint64)ask, (uint64)(h->arena_used - h->arena_start));
 		return false;
+	}
 	if(ask < HeapAllocChunk && HeapAllocChunk <= h->arena_end - h->arena_used)
 		ask = HeapAllocChunk;
 
@@ -191,9 +193,11 @@ MHeap_Grow(MHeap *h, uintptr npage)
 			ask = npage<<PageShift;
 			v = runtime·MHeap_SysAlloc(h, ask);
 		}
-		if(v == nil)
+		if(v == nil) {
+			runtime·printf("runtime: out of memory: operating system refused %D-byte allocation\n", (uint64)ask);
 			return false;
 		}
+	}
 	mstats.heap_sys += ask;
 
 	// Create a fake "in use" span and free it, so that the