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

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

Harmonize the newly ported test unit-tests/t-reftable-stack.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'.
- Function pointers should be passed as 'func' and not '&func'.

While at it, remove initialization for those variables that are
re-used multiple times, like loop variables.

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