compress/flate: Fixed two panics on bad data

I used just enough of the data provided by Matt in Issue 5915 to trigger
issue 5915. As luck would have it, using slightly less of it triggered
issue 5962.

Fixes #5915.
Fixes #5962.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12288043

src/pkg/compress/flate/flate_test.go

@@ -24,3 +24,26 @@ func TestUncompressedSource(t *testing.T) {
 		t.Errorf("output[0] = %x, want 0x11", output[0])
 	}
 }
+
+// The following test should not panic.
+func TestIssue5915(t *testing.T) {
+	bits := []int{4, 0, 0, 6, 4, 3, 2, 3, 3, 4, 4, 5, 0, 0, 0, 0, 5, 5, 6,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7, 8, 6, 0, 11, 0, 8, 0, 6, 6, 10, 8}
+	h := new(huffmanDecoder)
+	ok := h.init(bits)
+	if ok == true {
+		t.Fatalf("Given sequence of bits is bad, and should not succeed.")
+	}
+}
+
+// The following test should not panic.
+func TestIssue5962(t *testing.T) {
+	bits := []int{4, 0, 0, 6, 4, 3, 2, 3, 3, 4, 4, 5, 0, 0, 0, 0,
+		5, 5, 6, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 11}
+	h := new(huffmanDecoder)
+	ok := h.init(bits)
+	if ok == true {
+		t.Fatalf("Given sequence of bits is bad, and should not succeed.")
+	}
+}

src/pkg/compress/flate/inflate.go

@@ -125,6 +125,9 @@ func (h *huffmanDecoder) init(bits []int) bool {
 		if i == huffmanChunkBits+1 {
 			// create link tables
 			link := code >> 1
+			if huffmanNumChunks < link {
+				return false
+			}
 			h.links = make([][]uint32, huffmanNumChunks-link)
 			for j := uint(link); j < huffmanNumChunks; j++ {
 				reverse := int(reverseByte[j>>8]) | int(reverseByte[j&0xff])<<8
@@ -154,7 +157,11 @@ func (h *huffmanDecoder) init(bits []int) bool {
 				h.chunks[off] = chunk
 			}
 		} else {
-			linktab := h.links[h.chunks[reverse&(huffmanNumChunks-1)]>>huffmanValueShift]
+			value := h.chunks[reverse&(huffmanNumChunks-1)] >> huffmanValueShift
+			if value >= uint32(len(h.links)) {
+				return false
+			}
+			linktab := h.links[value]
 			reverse >>= huffmanChunkBits
 			for off := reverse; off < numLinks; off += 1 << uint(n-huffmanChunkBits) {
 				linktab[off] = chunk