Disallow empty import statements

libsolidity/interface/CompilerStack.cpp

@@ -681,6 +681,8 @@ StringMap CompilerStack::loadMissingSources(SourceUnit const& _ast, std::string
 	for (auto const& node: _ast.nodes())
 		if (ImportDirective const* import = dynamic_cast<ImportDirective*>(node.get()))
 		{
+			solAssert(!import->path().empty(), "Import path cannot be empty.");
+
 			string importPath = dev::absolutePath(import->path(), _sourcePath);
 			// The current value of `path` is the absolute path as seen from this source file.
 			// We first have to apply remappings before we can store the actual absolute path

libsolidity/parsing/Parser.cpp

@@ -221,6 +221,8 @@ ASTPointer<ImportDirective> Parser::parseImportDirective()
 			fatalParserError("Expected import path.");
 		path = getLiteralAndAdvance();
 	}
+	if (path->empty())
+		fatalParserError("Import path cannot be empty.");
 	nodeFactory.markEndPosition();
 	expectToken(Token::Semicolon);
 	return nodeFactory.createNode<ImportDirective>(path, unitAlias, move(symbolAliases));