Improve the protection of empty regular expressions (improves issue #61).

volatility/framework/layers/scanners/multiregexp.py

@@ -33,13 +33,15 @@ class MultiRegexp(object):
         self._pattern_strings.append(pattern)
 
     def preprocess(self) -> None:
+        if not self._pattern_strings:
+            raise ValueError("No strings to compile into a regular expression")
         self._regex = re.compile(b'|'.join(map(re.escape, self._pattern_strings)))
 
     def search(self, haystack: bytes) \
             -> Generator[Tuple[int, Union[str, bytes]], None, None]:
         if not isinstance(haystack, bytes):
             raise TypeError("Search haystack must be a byte string")
-        if not self._regex:
+        if not self._regex.pattern:
             raise ValueError("MultiRegexp cannot be used with an empty set of search strings")
         for match in re.finditer(self._regex, haystack):
             yield (match.start(0), match.group())