fix NULL ptr arithmetic in lz4:1680

only do arithmetic if offset > 0

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/lz4.c b/lib/lz4.c
index 106dd585..96599622 100644
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -1677,15 +1677,18 @@ int LZ4_compress_forceExtDict (LZ4_stream_t* LZ4_dict, const char* source, char*
 int LZ4_saveDict (LZ4_stream_t* LZ4_dict, char* safeBuffer, int dictSize)
 {
     LZ4_stream_t_internal* const dict = &LZ4_dict->internal_donotuse;
-    const BYTE* const previousDictEnd = dict->dictionary + dict->dictSize;
 
-    DEBUGLOG(5, "LZ4_saveDict : dictSize=%i, safeBuffer=%p, prevDictEnd=%p", dictSize, safeBuffer, previousDictEnd);
+    DEBUGLOG(5, "LZ4_saveDict : dictSize=%i, safeBuffer=%p", dictSize, safeBuffer);
 
     if ((U32)dictSize > 64 KB) { dictSize = 64 KB; } /* useless to define a dictionary > 64 KB */
     if ((U32)dictSize > dict->dictSize) { dictSize = (int)dict->dictSize; }
 
     if (safeBuffer == NULL) assert(dictSize == 0);
-    if (dictSize > 0) memmove(safeBuffer, previousDictEnd - dictSize, dictSize);
+    if (dictSize > 0) {
+        const BYTE* const previousDictEnd = dict->dictionary + dict->dictSize;
+        assert(dict->dictionary);
+        memmove(safeBuffer, previousDictEnd - dictSize, dictSize);
+    }
 
     dict->dictionary = (const BYTE*)safeBuffer;
     dict->dictSize = (U32)dictSize;