encoding.c: Don't crash when passing a NULL string to LookupMnem.

src/libespeak-ng/mnemonics.c

 int LookupMnem(MNEM_TAB *table, const char *string)
 {
 	while (table->mnem != NULL) {
-		if (strcmp(string, table->mnem) == 0)
+		if (string && strcmp(string, table->mnem) == 0)
 			return table->value;
 		table++;
 	}

tests/encoding.c

 void
 test_unknown_encoding()
 {
-	printf("testing unknown encoding\n");
+	printf("testing unknown encodings\n");
 
+	assert(espeak_ng_EncodingFromName(NULL) == ESPEAKNG_ENCODING_UNKNOWN);
 	assert(espeak_ng_EncodingFromName("") == ESPEAKNG_ENCODING_UNKNOWN);
 	assert(espeak_ng_EncodingFromName("abcxyz") == ESPEAKNG_ENCODING_UNKNOWN);
 	assert(espeak_ng_EncodingFromName("US") == ESPEAKNG_ENCODING_UNKNOWN); // wrong case