Fix conditional jump on unitialised value

This fixes a valgrind detected error. We check that
the number of real max sectors is greater than zero before
incrementing the value by 1. However ocassionaly the ioctl
call may not be able to obtain the dco and therefore the
ioctl data block is never populated. By zeroing the data
block prior to use and if it is not populated by the ioctl
call then the calculated real max sectors will be
zero and no increment will occur which is what we want.

src/hpa_dco.c

@@ -722,6 +722,9 @@ u64 nwipe_read_dco_real_max_sectors( char* device )
     unsigned char buffer[LBA_SIZE];  // Received data block
     unsigned char sense_buffer[SENSE_BUFFER_SIZE];  // Sense data
 
+    /* Zero the data block prior to use */
+    memset( buffer, 0, LBA_SIZE );
+
     /* three characters represent one byte of sense data, i.e
      * two characters and a space "01 AE 67"
      */