Fix issue #492: Handle GL implementations with 0 extensions

The get_exts() function incorrectly treated implementations with 0 extensions as an error case. This fix distinguishes between 'no extensions needed' (valid) and 'memory allocation failure' (error). - Modified glad/generator/c/templates/gl.c template logic - Separate handling for num_exts_i == 0 vs num_exts_i > 0 cases - Maintains full backward compatibility Fixes #492