Added null-ptr check in Colour::use()

include/internal/catch_console_colour.cpp

@@ -222,8 +222,14 @@ namespace Catch {
 
     void Colour::use( Code _colourCode ) {
         static IColourImpl* impl = platformColourInstance();
+        // Strictly speaking, this cannot possibly happen.
+        // However, under some conditions it does happen (see #1626),
+        // and this change is small enough that we can let practicality
+        // triumph over purity in this case.
+        if (impl != NULL) {
             impl->use( _colourCode );
         }
+    }
 
     std::ostream& operator << ( std::ostream& os, Colour const& ) {
         return os;