instead of current working directory
so, the fuzzer can be launched from anywhere

- fix a buffer overflow in ucd_tolower leading to failure when
compiling with address sanitizer
- force the use of C++ compiler for espeak-ng
- adding a malloc to have a null-terminated string in the fuzz target
- setting (but not overwriting) ESPEAK_DATA_PATH environment
variable inside the fuzz target

https://github.com/espeak-ng/espeak-ng/issues/407