Oss-fuzz integration

- 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

src/ucd-tools/src/case.c

@@ -2841,7 +2841,7 @@ codepoint_t ucd_tolower(codepoint_t c)
 {
 	int begin = 0;
 	int end   = sizeof(case_conversion_data)/sizeof(case_conversion_data[0]);
-	while (begin <= end)
+       while (begin < end)
 	{
 		int pos = (begin + end) / 2;
 		const struct case_conversion_entry *item = (case_conversion_data + pos);