Use the C_ARRAY_LENGTH.

Use the C_ARRAY_LENGTH.

Use the C_ARRAY_LENGTH instead of sizeof(c_array)/sizeof(c_array[0])
or sizeof(c_array)/sizeof(type_of_element_in_c_array), and C_ARRAY_END
for c_array + C_ARRAY_LENGTH(c_array).

While doing this found potential off-by-one error in sv.c:Perl_sv_magic:
how > C_ARRAY_LENGTH(PL_magic_data)
should probably have been
how >= C_ARRAY_LENGTH(PL_magic_data)
No tests fail, but this seems to be more of an internal sanity check.