Makefile: added variable SCANBUILD

so that a different version of scan-build can be selected

lib/decompress/zstd_decompress.c

@@ -542,6 +542,7 @@ size_t ZSTD_getcBlockSize(const void* src, size_t srcSize,
 static size_t ZSTD_copyRawBlock(void* dst, size_t dstCapacity,
                           const void* src, size_t srcSize)
 {
+    if (dst==NULL) return ERROR(dstSize_tooSmall);
     if (srcSize > dstCapacity) return ERROR(dstSize_tooSmall);
     memcpy(dst, src, srcSize);
     return srcSize;