re: issue #401

It turns out that the chunksize used in the
ncio-related code must be a multiple of eight in
size.  Both memio.c and mmapio.c were
potentially violating this constraint.

See also pr #400

libsrc/memio.c

@@ -385,6 +385,10 @@ fprintf(stderr,"memio_open: initial memory: %lu/%lu\n",(unsigned long)memio->mem
  /* Use half the filesize as the blocksize ; why? */
  sizehint = filesize/2;
 
+ /* sizehint must be multiple of 8 */
+ sizehint = (sizehint / 8) * 8;
+ if(sizehint < 8) sizehint = 8;
+
  fd = nc__pseudofd();
  *((int* )&nciop->fd) = fd;
 

libsrc/mmapio.c

@@ -373,6 +373,10 @@ fprintf(stderr,"mmapio_open: initial memory: %lu/%lu\n",(unsigned long)mmapio->m
  /* Use half the filesize as the blocksize */
  sizehint = filesize/2;
 
+ /* sizehint must be multiple of 8 */
+ sizehint = (sizehint / 8) * 8;
+ if(sizehint < 8) sizehint = 8;
+
  fd = nc__pseudofd();
  *((int* )&nciop->fd) = fd;