fuzzer: test env variable ESPEAK_DATA_PATH

before overwriting it with a default value for oss-fuzz

tests/ssml-fuzzer.c

 
 extern int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 	if (!initialized) {
-		setenv("ESPEAK_DATA_PATH",filepath,0);
+		const char *hasDataPath = getenv("ESPEAK_DATA_PATH");
+		if (!hasDataPath) {
+			setenv("ESPEAK_DATA_PATH",filepath,0);
+		}
 		espeak_Initialize(AUDIO_OUTPUT_SYNCHRONOUS, 0, NULL, 0);
 		espeak_SetSynthCallback(SynthCallback);
 		initialized = 1;