cgraph: fix: handle allocation failures in 'agcanon' and friends

cgraph: fix: handle allocation failures in 'agcanon' and friends

There are a number of cgraph API functions that use an internally managed buffer
to save the caller from having to allocate space themselves. Failure to expand
this buffer was being silently ignored, resulting in messy crashes when memory
was exhausted. These failures are now checked for and `agcanon` and `agcanonStr`
return `NULL` on allocation failure. `agwrite` returns `EOF` on allocation
failure.