cmTsVPrintfP.c: cmTsVPrintfP() added assert().

cmText.c

   va_list     vl1;
   va_copy(vl1,vl);
 
-  unsigned     n = vsnprintf(NULL,0,fmt,vl);
+  int     n = vsnprintf(NULL,0,fmt,vl);
+  assert(n != -1);
 
   s = cmMemResize(cmChar_t,s,n+1);