VLAs can be expensive on stack usage due to stack alignment requirements,
and also the fact that extra local variables are needed to track the
dynamic size of the stack.  So using fixed-size arrays when possible can
help to reduce code size and stack usage.

In this particular case, the maximum value of n_args in the VLA is 2 and so
it's more efficient to just allocate this array with a fixed size.  This
reduces code size by around 30 bytes on Thumb2 and Xtensa archs.  It also
reduces total stack usage of the function: on Thumb2 the usage with VLA is
between 40 and 48 bytes, which is reduced to 32; on Xtensa, VLA usage is
between 64 and 80 bytes, reduced to 32; on x86-64 it's at least 88 bytes
reduced to 80.