avoid computing 0 offsets from null pointers

Similar work in the kernel:
https://patchwork.kernel.org/patch/11351499/

UBsan (+clang-10) complains about doing pointer arithmetic (adding 0)
to a nullpointer.

This patch is tested with clang-10+ubsan

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/lz4.c b/lib/lz4.c
index 82ab490..2f7880a 100644
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -819,7 +819,7 @@ LZ4_FORCE_INLINE int LZ4_compress_generic(
 
     int const maybe_extMem = (dictDirective == usingExtDict) || (dictDirective == usingDictCtx);
     U32 const prefixIdxLimit = startIndex - dictSize;   /* used when dictDirective == dictSmall */
-    const BYTE* const dictEnd = dictionary + dictSize;
+    const BYTE* const dictEnd = dictionary ? dictionary + dictSize : dictionary;
     const BYTE* anchor = (const BYTE*) source;
     const BYTE* const iend = ip + inputSize;
     const BYTE* const mflimitPlusOne = iend - MFLIMIT + 1;
@@ -827,7 +827,7 @@ LZ4_FORCE_INLINE int LZ4_compress_generic(
 
     /* the dictCtx currentOffset is indexed on the start of the dictionary,
      * while a dictionary in the current context precedes the currentOffset */
-    const BYTE* dictBase = (dictDirective == usingDictCtx) ?
+    const BYTE* dictBase = !dictionary ? NULL : (dictDirective == usingDictCtx) ?
                             dictionary + dictSize - dictCtx->currentOffset :
                             dictionary + dictSize - startIndex;