Fix one of two flexible arrays in FTS3.

FossilOrigin-Name: ddfa87c17906ecf7fd5639a87bbfa9a87d17ab688159acd2fd80cc5b6f25f09b

ext/fts3/fts3_snippet.c

@@ -111,6 +111,10 @@ struct MatchinfoBuffer {
   u32 aMatchinfo[1];
 };
 
+/* Size (in bytes) of a MatchinfoBuffer sufficient for N elements */
+#define SZ_MATCHINFOBUFFER(N) \
+            (offsetof(MatchinfoBuffer,aMatchinfo)+((((N)+1)/2)*sizeof(u64))
+
 
 /*
 ** The snippet() and offsets() functions both return text values. An instance