Fix seq.setLen initialisation in VM (#6224)

tests/vm/tseq_badinit.nim

@@ -0,0 +1,58 @@
+
+type
+  AObj = object
+    i: int
+    d: float
+  ATup = tuple
+    i: int
+    d: float
+  MyEnum = enum
+    E01, E02, E03
+  Myrange = range[0..10]
+
+  MyProc = proc (x: int): bool
+  MyInt = distinct int
+  MyAlias = MyInt
+  MySet = set[char]
+  MyArray = array[4, char]
+  MySeq = seq[string]
+
+template test(typename, default: untyped) =
+  proc `abc typename`(): seq[typename] =
+    result = newSeq[typename]()
+    result.add(default)
+    result.setLen(3)
+    for i in 0 .. <2:
+      result[i] = default
+
+  const constval = `abc typename`()
+  doAssert(constval == `abc typename`())
+
+  proc `arr typename`(): array[4, typename] =
+    for i in 0 .. <2:
+      result[i] = default
+  const constarr = `arr typename`()
+  doAssert(constarr == `arr typename`())
+
+proc even(x: int): bool = x mod 2 == 0
+proc `==`(x, y: MyInt): bool = ord(x) == ord(y)
+proc `$`(x: MyInt): string = $ord(x)
+proc `$`(x: proc): string =
+  if x.isNil: "(nil)" else: "funcptr"
+
+test(int, 0)
+test(uint, 0)
+test(float, 0.1)
+test(char, '0')
+test(bool, false)
+test(uint8, 2)
+test(string, "data")
+test(MyProc, even)
+test(MyEnum, E02)
+test(AObj, AObj())
+test(ATup, (i:11, d:9.99))
+test(Myrange, 4)
+test(MyInt, MyInt(4))
+test(MyAlias, MyAlias(4))
+test(MyArray, ['0','1','2','3'])
+test(MySeq, @["data"])