Fix heap corruption in smb_getattachment() for blank attachments

MSVC detected heap corruption from this function when the attachment was 0-bytes in length. Good catch.

src/smblib/smbtxt.c

@@ -545,7 +545,7 @@ uint8_t* smb_getattachment(smbmsg_t* msg, char* buf, char* filename, size_t file
 	if(strStartsWith_i(msg->content_type, "multipart/") > 0) {
 		txt = mime_getpart(buf, msg->content_type, /* match-type: */NULL, 0, &xfer_encoding, /* charset: */NULL
 			,/* attachment: */filename, filename_len, index);
-		if(txt != NULL && xfer_encoding == CONTENT_TRANFER_ENCODING_BASE64) {
+		if(txt != NULL && *txt && xfer_encoding == CONTENT_TRANFER_ENCODING_BASE64) {
 			size_t len = strlen(txt);
 			memmove(buf, txt, len + 1);
 			int result = b64_decode(buf, len, buf, len);