inpgval.c, INPgetValue(), use INPgetNetTok() instead of INPgetTok()

To fix a bug reported by Clyde Shappee in
  Subject: Problem with pole zero analysis and net names
  https://sourceforge.net/p/ngspice/discussion/127605/thread/14338e7e/

When using node names with a leading slash '/'
  on the "pz" analysis command line.

src/spicelib/parser/inpgval.c

@@ -58,7 +58,7 @@ IFvalue *INPgetValue(CKTcircuit *ckt, char **line, int type, INPtables * tab)
     } else if (type == IF_FLAG) {
 	temp.iValue = 1;
     } else if (type == IF_NODE) {
-	INPgetTok(line, &word, 1);
+	INPgetNetTok(line, &word, 1);
 	INPtermInsert(ckt, &word, tab, &(temp.nValue));
     } else if (type == IF_INSTANCE) {
 	INPgetTok(line, &word, 1);