Fix problems with compilation in C++14 (GCC 6.x). Changes in iostream library
caused that comparison of istream to 0 or NULL is unavailable.
Gentoo bug: https://bugs.gentoo.org/show_bug.cgi?id=594332

--- a/src/interface.cpp
+++ b/src/interface.cpp
@@ -1340,7 +1340,7 @@
 
 
 	int isExistedFile_ = 1;
-	if (existedFile_ == 0)
+	if (!existedFile_)
 		isExistedFile_ = 0;
 
 	existedFile_.close ();
--- a/src/iqp.cpp
+++ b/src/iqp.cpp
@@ -508,7 +508,7 @@
 		if (in_pam.tree_file != NULL) {
 			std::ifstream userTreeFile_;
 			userTreeFile_.open (in_pam.tree_file);
-			if (userTreeFile_ != 0) {
+			if (userTreeFile_) {
 				initialTree_.readFile (in_pam.tree_file);
 				initialTree_.createUrTree ();
 				hasInitTree = true;
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -118,7 +118,7 @@
 
 	ifstream in;
 	in.open (boottree_file_name.c_str());
-	if (in == 0)
+	if (!in)
 		Utl::announceError ("Cannot open the user tree file ...");
 
 	int num_tree = 0;
--- a/src/usertree.cpp
+++ b/src/usertree.cpp
@@ -94,7 +94,7 @@
 void UserTree::readFile (const char *userTreeFile) {
 	ifstream in;
 	in.open (userTreeFile);
-	if (in == 0)
+	if (!in)
 		Utl::announceError ("Cannot open the user tree file ...");
 
 	readFile(in);