t: harmonize t-reftable-block.c with coding guidelines

t: harmonize t-reftable-block.c with coding guidelines

Harmonize the newly ported test unit-tests/t-reftable-block.c
with the following guidelines:
- Single line 'for' statements must omit curly braces.
- Structs must be 0-initialized with '= { 0 }' instead of '= { NULL }'.
- Array sizes and indices should preferably be of type 'size_t'and
  not 'int'.
- Return code variable should preferably be named 'ret', not 'n'.

Mentored-by: Patrick Steinhardt <redacted>
Mentored-by: Christian Couder <redacted>
Signed-off-by: Chandra Pratap <redacted>
Signed-off-by: Junio C Hamano <redacted>