cpitf.c, cp_istrue(), avoid surplus Warning Message when an `if' condition expands to nothing

When in expression
  if $var ...
the variable `var' was undefined, then
ngspice printed a surplus warning message
>  Warning: NULL arithmetic expression
in addition to the error message
>  Error: var: no such variable.

ngspice continues to process the conditional construct
  and evaluates the condition as "FALSE"

examples/control_structs/if-test-1.cir

@@ -36,7 +36,6 @@ end
 echo
 
 echo "expect Error: testvar_d: no such variable."
-echo "expect Warning: NULL arithmetic expression"
 * in cp_istrue(): wl!=NULL then wl==NULL names==NULL v==NULL
 if $testvar_d
   echo "FAIL: you should not see this"

src/frontend/cpitf.c

@@ -306,6 +306,9 @@ cp_istrue(wordlist *wl)
     /* First do all the csh-type stuff here... */
     wl = wl_copy(wl);
     wl = cp_variablesubst(wl);
+    /* return FALSE if this did not expand to anything */
+    if (!wl)
+        return FALSE;
     wl = cp_bquote(wl);
     cp_striplist(wl);