lib: handle stream extraction failure with fail() check

ensure read failure is detected if file is empty or contains invalid
data. Closes #179.

Signed-off-by: Siddhesh Dharme <siddheshdharme18@gmail.com>

Author: sdharme

src/lib.cpp

@@ -199,6 +199,11 @@ int read_sysfs(const string &filename, bool *ok)
 	try
 	{
 		file >> i;
+        if(file.fail()){
+            if(ok)
+                *ok = false;
+            return 0;
+        }
 		if (ok)
 			*ok = true;
 	} catch (std::exception &exc) {