MDEV-15420 Wrong result for CAST from TIME or DATETIME with zero inte…

…ger part and non-zero microseconds to DECIMAL(X,Y)

The loop in ull2dec() does not iterate if "from" is zero,
so to->intg got erroneously set to 0 instead of 1.
Because if this, my_decimal2seconds() wrote the fractional
part into a wrong buf[x].

Catching the special case with zero "from" and properly initialize "to"
using decimal_make_zero().

mysql-test/r/type_decimal.result

@@ -1019,3 +1019,19 @@ cast('-0.0' as decimal(5,1)) < 0
 #
 # End of 5.5 tests
 #
+#
+# Start of 10.2 tests
+#
+#
+# MDEV-15420 Wrong result for CAST from TIME or DATETIME with zero integer part and non-zero microseconds to DECIMAL(X,Y)
+#
+SET sql_mode='';
+SELECT
+CAST(TIMESTAMP'0000-00-00 00:00:00.123456' AS DECIMAL(10,6)) AS c1,
+CAST(TIME'00:00:00.123456' AS DECIMAL(10,6)) AS c2;
+c1	c2
+0.123456	0.123456
+SET sql_mode=DEFAULT;
+#
+# End of 10.2 tests
+#

mysql-test/t/type_decimal.test

@@ -612,3 +612,21 @@ select cast('-0.0' as decimal(5,1)) < 0;
 --echo #
 --echo # End of 5.5 tests
 --echo #
+
+--echo #
+--echo # Start of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-15420 Wrong result for CAST from TIME or DATETIME with zero integer part and non-zero microseconds to DECIMAL(X,Y)
+--echo #
+
+SET sql_mode=''; # Allow zero date
+SELECT
+  CAST(TIMESTAMP'0000-00-00 00:00:00.123456' AS DECIMAL(10,6)) AS c1,
+  CAST(TIME'00:00:00.123456' AS DECIMAL(10,6)) AS c2;
+SET sql_mode=DEFAULT;
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

strings/decimal.c

@@ -1000,6 +1000,12 @@ static int ull2dec(ulonglong from, decimal_t *to)
 
   sanity(to);
 
+  if (!from)
+  {
+    decimal_make_zero(to);
+    return E_DEC_OK;
+  }
+
   for (intg1=1; from >= DIG_BASE; intg1++, from/=DIG_BASE) {}
   if (unlikely(intg1 > to->len))
   {