added test for multiplication and relative fix

Author: pietroppeter

src/bigints.nim

@@ -476,6 +476,9 @@ proc multiplicationInt(a: var BigInt, b: BigInt, c: int32) =
 
 # This doesn't work when a = b
 proc multiplication(a: var BigInt, b, c: BigInt) =
+  if b.isZero or c.isZero:
+    a = zero
+    return
   let
     bl = b.limbs.len
     cl = c.limbs.len
@@ -487,9 +490,6 @@ proc multiplication(a: var BigInt, b, c: BigInt) =
   else:
     unsignedMultiplication(a, b, c, bl, cl)
 
-  if a.limbs == @[0'u32]:
-    return
-
   if Negative in b.flags:
     if Negative in c.flags:
       a.flags.excl(Negative)

tests/tester.nim

@@ -201,3 +201,19 @@ test "empty limbs when uninitialized (https://github.com/def-/nim-bigints/issues
   check -bigOne - -zeroEmpty == -bigOne # ok
   check zeroEmpty - -bigOne == bigOne # ok
   check -zeroEmpty - -bigOne == bigOne # ok
+
+  # multiplication
+  check zeroEmpty * 1.int32 == zero
+  check -zeroEmpty * 1.int32 == zero
+  check zeroEmpty * one == zero
+  check -zeroEmpty * one == zero
+  check one * zeroEmpty == zero
+  check one * -zeroEmpty == zero
+
+  # https://github.com/def-/nim-bigints/issues/26
+  block:
+    var
+      a: BigInt
+      b: BigInt = 12.initBigInt
+
+    check a*b == 0