Avoid writing uninitialised data.

b%dims wasn't being initialised in some subroutines, but is still
written to the block header.

Although this bug only appears for blocks which don't really need
b%dims, it was breaking sdfdiff.

I've only applied this fix to one file that contained the subroutine
that was causing a problem for me. It might need to be applied to other
files too.
3 jobs for bugfix/initialise-dims in 0 seconds (queued for 1937 minutes and 13 seconds)
Status Job ID Name Coverage
  Test
failed #3606
osx
test_osx

failed #3604
gfortran ubuntu
test_ubuntu_gfortran

failed #3605
ubuntu intel
test_ubuntu_intel

 
Name Stage Failure
failed
test_osx Test There is an unknown failure, please try again
No job trace
failed
test_ubuntu_intel Test There is an unknown failure, please try again
No job trace
failed
test_ubuntu_gfortran Test There is an unknown failure, please try again
No job trace