SR-13015: Decimal calculation incorrect on Linux

Fix two old typos in the overflow check in integerMultiply(), which was screwing up
Decimal calculations under certain conditions.

Author: finagolfin

Sources/Foundation/Decimal.swift

@@ -1025,7 +1025,7 @@ fileprivate func integerMultiply<T:VariableLengthNumber>(_ big: inout T,
                 accumulator = UInt32(carry) + bigij + UInt32(right[j]) * UInt32(left[i])
                 carry = UInt16(truncatingIfNeeded:accumulator >> 16)
                 big[i+j] = UInt16(truncatingIfNeeded:accumulator)
-            } else if carry != 0 || (right[j] == 0 && left[j] == 0) {
+            } else if carry != 0 || !(right[j] == 0 && left[i] == 0) {
                 return .overflow
             }
         }

Tests/Foundation/Tests/TestDecimal.swift

@@ -99,6 +99,10 @@ class TestDecimal: XCTestCase {
         XCTAssertEqual(.lossOfPrecision, NSDecimalAdd(&result, &one, &addend, .plain), "1 + 1e-39")
         XCTAssertEqual("1", result.description)
         XCTAssertEqual(.orderedSame, NSDecimalCompare(&one, &result), "1 + 1e-39")
+
+        let a = Decimal(string: "498.7509045")!
+        let b = Decimal(string: "8.453441368210501065891847765109162027")!
+        XCTAssertEqual(a + b, Decimal(string: "507.2043458682105010658918477651091")!)
     }
 
     func test_BasicConstruction() {