Verify that if the index.to field is non-zero or there's a to_ext header field,

that they match. This will catch a QWK networking bug in previous
Synchronet versions where the index.to was non-zero (the number of
the QWKnet node account), but there was no hdr.to_ext field, thus the index
could not be recreated from the information in the header (e.g. by fixsmb).
This was actually an intentional design decision at the time, but in hindsight,
it's a bug (the index file is not supposed to contain any unique information).

src/sbbs3/chksmb.c

@@ -437,7 +437,8 @@ int main(int argc, char **argv)
 					subjcrc++; 
 				}
 				if(smb.status.attr&SMB_EMAIL 
-					&& msg.from_ext && msg.idx.from!=atoi(msg.from_ext)) {
+					&& (msg.from_ext!=NULL || msg.idx.from) 
+					&& (msg.from_ext==NULL || msg.idx.from!=atoi(msg.from_ext))) {
 					fprintf(stderr,"%sFrom extension mismatch\n",beep);
 					msgerr=TRUE;
 					if(extinfo)
@@ -457,7 +458,8 @@ int main(int argc, char **argv)
 					fromcrc++; 
 				}
 				if(smb.status.attr&SMB_EMAIL 
-					&& msg.to_ext && msg.idx.to!=atoi(msg.to_ext)) {
+					&& (msg.to_ext!=NULL || msg.idx.to) 
+					&& (msg.to_ext==NULL || msg.idx.to!=atoi(msg.to_ext))) {
 					fprintf(stderr,"%sTo extension mismatch\n",beep);
 					msgerr=TRUE;
 					if(extinfo)