Added NULL test for glGetStringi.

src/context.c

@@ -595,9 +595,10 @@ GLFWAPI int glfwExtensionSupported(const char* extension)
 
         for (i = 0;  i < count;  i++)
         {
-             if (strcmp((const char*) window->GetStringi(GL_EXTENSIONS, i),
+            const char* en = (const char*) window->GetStringi(GL_EXTENSIONS, i);
-                         extension) == 0)
+            if (en != NULL)
             {
+                if (strcmp(en, extension) == 0)
                     return GL_TRUE;
             }
         }