mxfdec: Fix the error handling for when strftime fails

The str variable is a char ** here.
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 6448f15a)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -1976,7 +1976,7 @@ static int mxf_timestamp_to_str(uint64_t timestamp, char **str)
     if (!*str)
         return AVERROR(ENOMEM);
     if (!strftime(*str, 32, "%Y-%m-%d %H:%M:%S", &time))
-        str[0] = '\0';
+        (*str)[0] = '\0';
 
     return 0;
 }