chat_msg: fix strstr on non-null-terminated blob/text data

chat_core_attachment_download: use sqlite3_column_text for null-term
on_msg_inserted: copy data to local null-term buf before strstr
Both caused b64_decode to misread body from garbage memory

src/chat/chat_msg.c

@@ -541,8 +541,11 @@ void on_msg_inserted(struct DB_SYNC_INSTANCE* si, uint64_t record_ts, const char
 
     /* auto-download media if message contains media metadata */
     if (data && len > 0 && author != g_cc.my_node_id) {
-        const char* ct = strstr(data, "\"ct\":\"");
-        const char* dpos = strstr(data, "\"d\":\"");
+        /* data from db_sync may not be null-terminated; copy to local buffer */
+        char buf[4096]; size_t blen = len < sizeof(buf) - 1 ? len : sizeof(buf) - 1;
+        memcpy(buf, data, blen); buf[blen] = '\0';
+        const char* ct = strstr(buf, "\"ct\":\"");
+        const char* dpos = strstr(buf, "\"d\":\"");
         if (ct && dpos) {
             const char* d_start = dpos + 5;
             char body[4096]; size_t bi = 0;
@@ -560,7 +563,7 @@ void on_msg_inserted(struct DB_SYNC_INSTANCE* si, uint64_t record_ts, const char
             }
             if (bfi == 0) snprintf(base_filename, sizeof(base_filename), "file");
 
-            const uint8_t* sig_field = strstr(data, "\"sig\":\"");
+            const uint8_t* sig_field = (const uint8_t*)strstr(buf, "\"sig\":\"");
             uint8_t author_sig[64] = {0};
             if (sig_field) {
                 /* read author_signature from DB — find by ts in on_msg_inserted we don't have sig
@@ -616,7 +619,7 @@ void chat_core_attachment_download(const char* channel_id, int64_t msg_id) {
         sqlite3_finalize(st);
         return;
     }
-    const uint8_t* jdata = (const uint8_t*)sqlite3_column_blob(st, 0);
+    const uint8_t* jdata = sqlite3_column_text(st, 0);
     int jlen = sqlite3_column_bytes(st, 0);
     int64_t db_ts = sqlite3_column_int64(st, 1);
     const uint8_t* sig_blob = (const uint8_t*)sqlite3_column_blob(st, 2);