Suppress warning from older compilers.

Commit 8af1624e3 introduced a warning about possibly returning
without a value, on compilers that don't realize that ereport(ERROR)
doesn't return.  Tweak the code to avoid that.

Per buildfarm.  Back-patch to 9.6, like the aforesaid commit.

src/backend/tsearch/spell.c

@@ -1173,8 +1173,7 @@ getAffixFlagSet(IspellDict *Conf, char *s)
 			ereport(ERROR,
 					(errcode(ERRCODE_CONFIG_FILE_ERROR),
 					 errmsg("invalid affix alias \"%s\"", s)));
-		else
-			return VoidString;
+		return VoidString;
 	}
 	else
 		return s;