Cherry pick underflow fix.

third_party/brotli/dec/decode.c

Index: third_party/brotli/dec/decode.c
diff --git a/third_party/brotli/dec/decode.c b/third_party/brotli/dec/decode.c
index bd0d2132d8f57a70b9979fd08cbe4dcf1c8eeedd..f0bb469d024250235be22eb5fd770e6b7c29a3cb 100644
--- a/third_party/brotli/dec/decode.c
+++ b/third_party/brotli/dec/decode.c
@@ -1672,6 +1672,10 @@ postReadDistance:
     uint8_t* copy_src = &s->ringbuffer[
         (pos - s->distance_code) & s->ringbuffer_mask];
     uint8_t* copy_dst = &s->ringbuffer[pos];
+    /* Check for possible underflow and clamp the pointer to 0. */
+    if (PREDICT_FALSE(s->ringbuffer_end < (const uint8_t*)0 + i)) {
+      ringbuffer_end_minus_copy_length = 0;
+    }
     /* update the recent distances cache */
     s->dist_rb[s->dist_rb_idx & 3] = s->distance_code;
     ++s->dist_rb_idx;