Fix fann_error stack overflow

There was a bug in fann_error function. The `errstr` string was limited by FANN_ERRSTR_MAX (128) characters. It caused string overflow for all error messages with longer file path as a parameter (`FANN_E_CANT_READ_CONFIG`, `FANN_E_CANT_OPEN_CONFIG_R`...). It obviously resulted in the heap corruption...

In addition there was a memory leak for `errstr`  when `errdat` was NULL.

This patch fixes both bugs. I checked it with valgrind and also run all my test tests that I have in my php fann ext.