tree-wide: use STRLEN() to allocate buffer of constant size

tree-wide: use STRLEN() to allocate buffer of constant size

Using strlen() to declare a buffer results in a variable-length array,
even if the compiler likely optimizes it to be a compile time constant.

When building with -Wvla, certain versions of gcc complain about such
buffers. Compiling with -Wvla has the advantage of preventing variably
length array, which defeat static asserts that are implemented by
declaring an array of negative length.