niconv.c, NIconvTest(), NaN in CKTrhs denotes non-convergence

emit a warning message, but only if ft_ngdebug is set

src/maths/ni/niconv.c

@@ -14,6 +14,8 @@ Author: 1985 Thomas L. Quarles
 #include "ngspice/cktdefs.h"
 #include "ngspice/smpdefs.h"
 
+extern bool ft_ngdebug;
+
 
 int
 NIconvTest(CKTcircuit *ckt)
@@ -38,6 +40,11 @@ NIconvTest(CKTcircuit *ckt)
         node = node->next;
         new =  ckt->CKTrhs [i] ;
         old =  ckt->CKTrhsOld [i] ;
+        if (isnan(new)) {
+            if (ft_ngdebug)
+                fprintf(stderr, "Warning: non-convergence, node %s is nan\n", CKTnodName(ckt, i));
+            return 1;
+        }
         if(node->type == SP_VOLTAGE) {
             tol =  ckt->CKTreltol * (MAX(fabs(old),fabs(new))) +
                     ckt->CKTvoltTol;