Save errno before using sprintf, ifstream.

std::ifstream in libstdc++ contains a bug, where it sets errno to zero.
To work around it, we manually save the errno before using std::ifstream
in debugger check, and reset it after we are done.

We also preventively save errno before using sprintf.

Fixes #835

include/reporters/catch_reporter_bases.hpp

@@ -9,6 +9,7 @@
 #define TWOBLUECUBES_CATCH_REPORTER_BASES_HPP_INCLUDED
 
 #include "../internal/catch_interfaces_reporter.h"
+#include "../internal/catch_errno_guard.hpp"
 
 #include <cstring>
 #include <cfloat>
@@ -27,6 +28,9 @@ namespace Catch {
             // + 1 for null terminator
             const size_t maxDoubleSize = DBL_MAX_10_EXP + 1 + 1 + 3 + 1;
             char buffer[maxDoubleSize];
+            
+            // Save previous errno, to prevent sprintf from overwriting it
+            ErrnoGuard guard;
 #ifdef _MSC_VER
             sprintf_s(buffer, "%.3f", duration);
 #else