Fix valgrind warning about write of uninitialized bytes in ScaleOffse…

…t filter (#3390)

src/H5Zscaleoffset.c

@@ -1283,6 +1283,9 @@ H5Z__filter_scaleoffset(unsigned flags, size_t cd_nelmts, const unsigned cd_valu
     }
     /* output; compress */
     else {
+        size_t used_bytes;
+        size_t unused_bytes;
+
         assert(nbytes == d_nelmts * p.size);
 
         /* before preprocess, convert to memory endianness order if needed */
@@ -1334,7 +1337,10 @@ H5Z__filter_scaleoffset(unsigned flags, size_t cd_nelmts, const unsigned cd_valu
         /* (Looks like an error in the original determination of how many
          *      bytes would be needed for parameters. - QAK, 2010/08/19)
          */
-        memset(outbuf + 13, 0, (size_t)8);
+        used_bytes = 4 + 1 + sizeof(unsigned long long);
+        assert(used_bytes <= size_out);
+        unused_bytes = size_out - used_bytes;
+        memset(outbuf + 13, 0, unused_bytes);
 
         /* special case: minbits equal to full precision */
         if (minbits == p.size * 8) {