naming convention detector: Fix is_mixed_case with _

slither/detectors/naming_convention/naming_convention.py

@@ -41,7 +41,7 @@ Solidity defines a [naming convention](https://solidity.readthedocs.io/en/v0.4.2
     def is_mixed_case_with_underscore(name):
         # Allow _ at the beginning to represent private variable
         # or unused parameters
-        return re.search('^[a-z_]([A-Za-z0-9]+)?_?$', name) is not None
+        return re.search('^[_]?[a-z]([A-Za-z0-9]+)?_?$', name) is not None
 
     @staticmethod
     def is_upper_case_with_underscores(name):