read_writev: avoid reading past the last iovec elem

If iovcnt is exhausted and the first vector element's operation
is satisfied, the while loop will read past the end of the
iovec array.  This triggers the address sanitizer and leads to
undefined program state.  Avoid reading too far.

src/util.c

@@ -790,7 +790,7 @@ int read_writev(int fd, struct iovec *iov, int iovcnt, int rw)
 	return 0;
       
       done += n;
-      while ((size_t)done >= iov[cur].iov_len)
+      while (cur < iovcnt && (size_t)done >= iov[cur].iov_len)
 	done -= iov[cur++].iov_len;
     }