Fixes Natspec parser error when whitespace is missing.

test/libsolidity/syntaxTests/natspec/docstring_parameter.sol

@@ -0,0 +1,11 @@
+contract C {
+    /**
+     * @dev a function
+     * @param _from 5 from address
+     * @param _5to   : to address
+     * @param value: token transfer amount
+     * @param value? token transfer amount
+     */
+    function f(address _from, address _5to, uint256 value) internal {
+    }
+}

test/libsolidity/syntaxTests/natspec/invalid/docstring_parameter.sol

@@ -0,0 +1,11 @@
+contract C {
+    /**
+     * @param 5value a value parameter
+     * @param _ a value parameter
+     */
+    function f(uint256 _5value, uint256 _value, uint256 value) internal {
+    }
+}
+// ----
+// DocstringParsingError: Documented parameter "" not found in the parameter list of the function.
+// DocstringParsingError: Documented parameter "_" not found in the parameter list of the function.