🔨 trying to fix the sanitizer error

Travis found an error with Clang 3.8's sanitizers, see https://travis-ci.org/nlohmann/json/jobs/256366699. Unfortunately, I cannot reproduce this error with clang version 6.0.0 (trunk 308825) locally. However, this seems to be an issue, because so far, we did not reset a value after moving from it.

src/json.hpp

@@ -3281,6 +3281,7 @@ class parser
                 while (true)
                 {
                     // parse value
+                    value = value_t::discarded;
                     parse_internal(keep, value);
                     if (keep and not value.is_discarded())
                     {