plug-ins: metadata-editor crashes when strtok() initialized with NULL.
When running strtok() the first time, it needs to be non-NULL so we must check for the string. This is even more important because NULL actually has a special meaning in strtok() to indicate further search on the same string, in a stateful way. So searching with NULL at first call was crashing the metadata editor plug-in in my case. I could also imagine it could have reused strings from previous searches, mixing metadata contents in some edge cases. Anyway that would be bad as well! While I was there, I also checked for non-null search string before strstr() calls, when there was not already such a check before. This function also requires non-NULL haystack argument. It feels like this code doesn't do much validity checks, and it's likely there are more similar issues. I haven't reviewed the whole code, only this part which was crashing here.
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