Fix unescape_string removing all backslashes

src/util/string.h

@@ -398,8 +398,7 @@ inline std::string wrap_rows(const std::string &from,
 
 
 /**
- * Removes all backslashes from a string that had been escaped (FormSpec strings)
- *
+ * Removes backslashes from an escaped string (FormSpec strings)
  */
 template <typename T>
 inline std::basic_string<T> unescape_string(std::basic_string<T> &s)
@@ -407,8 +406,11 @@ inline std::basic_string<T> unescape_string(std::basic_string<T> &s)
 	std::basic_string<T> res;
 
 	for (size_t i = 0; i < s.length(); i++) {
-		if (s[i] != '\\')
-			res += s[i];
+		if (s[i] == '\\')
+			i++;
+		if (i >= s.length())
+			break;
+		res += s[i];
 	}
 
 	return res;