delmail() re-writes the mail.sid file. Since I've seen some recent

mail.sid corruption approximately around midnight (when delmail()) is
called as part of the "DAILY:" mail maintenance, I'm suspicious of this
function and while I haven't found any bugs, I did find (and now fixed)
lack of error checking in calls to smb_fsetlength() and smb_fwrite().

src/sbbs3/mail.cpp

@@ -113,13 +113,20 @@ int sbbs_t::delmail(uint usernumber, int which)
 		l++; 
 	}
 	smb_rewind(smb.sid_fp);
-	smb_fsetlength(smb.sid_fp,0);
-	for(i=0;i<l;i++)
-		smb_fwrite(&smb,&idxbuf[i],sizeof(idxrec_t),smb.sid_fp);
+	if(smb_fsetlength(smb.sid_fp,0) != 0)
+		errormsg(WHERE, "truncating", smb.file, 0);
+	else {
+		for(i=0;i<l;i++) {
+			if(smb_fwrite(&smb,&idxbuf[i],sizeof(idxrec_t),smb.sid_fp) != sizeof(idxrec_t)) {
+				errormsg(WHERE, ERR_WRITE, smb.file, i);
+				break;
+			}
+		}
+		smb.status.total_msgs=i;
+		smb_putstatus(&smb);
+		smb_fflush(smb.sid_fp);
+	}
 	free(idxbuf);
-	smb.status.total_msgs=l;
-	smb_putstatus(&smb);
-	smb_fflush(smb.sid_fp);
 	smb_close_ha(&smb);
 	smb_close_da(&smb);
 	return removed;