bound binary-protocol datetime/time parameter reads to the declared length

get_param_length() guarantees only `length` bytes for a prepared-statement
parameter value sent over the binary protocol. set_param_datetime() reads
to[4..6] when length > 4 (needs 7 bytes) and sint4korr(to+7) when length > 7
(needs 11); set_param_time() reads sint4korr(to+8) when length > 8 (needs 12).

A client that sends an off-spec datetime length of 5, 6, 8, 9 or 10, or a
time length of 9, 10 or 11, makes the server read up to 3 bytes past the
validated value, an out-of-bounds read when the value ends at the packet
buffer boundary.

Gate the optional time and microsecond reads on the canonical encoding
lengths (>= 7 and >= 11 for datetime, >= 12 for time) so a short value falls
back to the next-shorter form instead of over-reading. Well-formed values
(datetime 4/7/11, time 8/12) parse exactly as before.

Author: jmestwa-coder

sql/sql_prepare.cc

@@ -578,7 +578,7 @@ void Item_param::set_param_time(uchar **pos, ulong len)
     tm.hour=   (uint) to[5] + day * 24;
     tm.minute= (uint) to[6];
     tm.second= (uint) to[7];
-    tm.second_part= (length > 8) ? (ulong) sint4korr(to+8) : 0;
+    tm.second_part= (length >= 12) ? (ulong) sint4korr(to+8) : 0;
     if (tm.hour > 838)
     {
       /* TODO: add warning 'Data truncated' here */
@@ -607,7 +607,7 @@ void Item_param::set_param_datetime(uchar **pos, ulong len)
     tm.year=   (uint) sint2korr(to);
     tm.month=  (uint) to[2];
     tm.day=    (uint) to[3];
-    if (length > 4)
+    if (length >= 7)
     {
       tm.hour=   (uint) to[4];
       tm.minute= (uint) to[5];
@@ -616,7 +616,7 @@ void Item_param::set_param_datetime(uchar **pos, ulong len)
     else
       tm.hour= tm.minute= tm.second= 0;
 
-    tm.second_part= (length > 7) ? (ulong) sint4korr(to+7) : 0;
+    tm.second_part= (length >= 11) ? (ulong) sint4korr(to+7) : 0;
   }
   else
     set_zero_time(&tm, MYSQL_TIMESTAMP_DATETIME);