[sparse] Only declare out of range if decompressed buffer is empty

ZX-1821 #done

Change-Id: Iedd0fe7cee0797b2e267fb1f4636171c570414fd
diff --git a/system/ulib/fvm/fvm-lz4.cpp b/system/ulib/fvm/fvm-lz4.cpp
index 1f617e3..d81c80b 100644
--- a/system/ulib/fvm/fvm-lz4.cpp
+++ b/system/ulib/fvm/fvm-lz4.cpp
@@ -146,7 +146,7 @@
 #endif
     size_t total_size = 0;
     if (compressed_) {
-        if (to_read_ == 0) {
+        if (out_buf_.is_empty() && to_read_ == 0) {
             // There is no more to read
             return ZX_ERR_OUT_OF_RANGE;
         }