Fix errors in numeric binary decoder

The size of the output buffer in numeric decoder is computed incorrectly
which may lead to stack corruption or access to unitialized memory.

This also fixes incorrect rendering of trailing zeros in some cases.

Fixes: #520
Fixes: #514

Author: elprans

asyncpg/pgproto

@@ -69,7 +69,6 @@ def _system_timezone():
         decimal.Decimal("-1.00000000000000"),
         decimal.Decimal("-2.00000000000000"),
         decimal.Decimal("1000000000000000.00000000000000"),
-        decimal.Decimal("-0.00000000000000"),
         decimal.Decimal(1234),
         decimal.Decimal(-1234),
         decimal.Decimal("1234000000.00088883231"),
@@ -111,7 +110,7 @@ def _system_timezone():
         decimal.Decimal("0.0"),
         decimal.Decimal("-1.0"),
         decimal.Decimal("1.0E-1000"),
-        decimal.Decimal("1.0E1000"),
+        decimal.Decimal("1E1000"),
         decimal.Decimal("0.000000000000000000000000001"),
         decimal.Decimal("0.000000000000010000000000001"),
         decimal.Decimal("0.00000000000000000000000001"),
@@ -141,6 +140,16 @@ def _system_timezone():
         decimal.Decimal("0.001"),
         decimal.Decimal("0.01"),
         decimal.Decimal("0.1"),
+        decimal.Decimal("0.10"),
+        decimal.Decimal("0.100"),
+        decimal.Decimal("0.1000"),
+        decimal.Decimal("0.10000"),
+        decimal.Decimal("0.100000"),
+        decimal.Decimal("0.00001000"),
+        decimal.Decimal("0.000010000"),
+        decimal.Decimal("0.0000100000"),
+        decimal.Decimal("0.00001000000"),
+        decimal.Decimal("1" + "0" * 117 + "." + "0" * 161)
     )),
     ('bytea', 'bytea', (
         bytes(range(256)),
@@ -502,6 +511,14 @@ async def test_standard_codecs(self):
                     else:
                         self.assertEqual(result, outputval, err_msg)
 
+                    if (typname == 'numeric' and
+                            isinstance(inputval, decimal.Decimal)):
+                        self.assertEqual(
+                            result.as_tuple(),
+                            outputval.as_tuple(),
+                            err_msg,
+                        )
+
             with self.subTest(sample=None, typname=typname):
                 # Test that None is handled for all types.
                 rsample = await st.fetchval(None)