Fix parsing issue involving effective dates

Cory Duplantis reported that "A specially crafted journal file can
cause [an] integer underflow resulting in code execution".  Cory
provided this test case:

    Expenses:Food:Groceries             $ 37.50  ; ] [=2004/01/01]

Note the ] that comes before [ after the ;.

This issue was reported and described in great detail by Cory Duplantis
of Cisco Talos.  This issue is known as TALOS-2017-0303 and has been
assigned CVE-2017-2807.  Cory's description can be found at
https://www.talosintelligence.com/vulnerability_reports/TALOS-2017-0303

Fixes #1722

doc/NEWS

@@ -42,6 +42,9 @@
 
 - Do not parse user-specified init-file twice
 
+- Fix parsing issue of effective dates (bug #1722, TALOS-2017-0303,
+  CVE-2017-2807)
+
 - Python: Removed double quotes from Unicode values.
 
 - Python: Ensure that parse errors produce useful RuntimeErrors

src/item.cc

@@ -152,7 +152,7 @@ void item_t::parse_tags(const char * p,
     if (const char * b = std::strchr(p, '[')) {
       if (*(b + 1) != '\0' &&
           (std::isdigit(*(b + 1)) || *(b + 1) == '=')) {
-        if (const char * e = std::strchr(p, ']')) {
+        if (const char * e = std::strchr(b, ']')) {
           char buf[256];
           std::strncpy(buf, b + 1, static_cast<std::size_t>(e - b - 1));
           buf[e - b - 1] = '\0';

test/regress/1722.test

@@ -0,0 +1,12 @@
+
+2003/12/20 Organic Co-op
+  Expenses:Food:Groceries             $ 37.50  ; ] [=2004/01/01]
+  Assets:Cash                         $-37.50
+
+test bal
+            $ -37.50  Assets:Cash
+             $ 37.50  Expenses:Food:Groceries
+--------------------
+                   0
+end test
+