MINOR: qpack: fix memory leak on huffman decoding

Remove an unneeded strdup invocation during QPACK huffman decoding. A
temporary storage buffer is passed by the function and exists after
decoding so no need to duplicate memory here.
diff --git a/src/qpack-dec.c b/src/qpack-dec.c
index 54e7f16..a67ecb7 100644
--- a/src/qpack-dec.c
+++ b/src/qpack-dec.c
@@ -311,7 +311,9 @@
 				}
 
 				qpack_debug_printf(stderr, " [name huff %d->%d '%s']", (int)length, (int)nlen, trash);
-				list[hdr_idx].v = ist(strdup(trash));
+				/* makes an ist from tmp storage */
+				b_add(tmp, nlen);
+				list[hdr_idx].v = ist2(trash, nlen);
 			}
 			/* XXX Value string XXX */
 			raw += length;