py/objstr: Protect against creating bytes(n) with n negative.

Prior to this patch uPy (on a 32-bit arch) would have severe issues when
calling bytes(-1): such a call would call vstr_init_len(vstr, -1) which
would then +1 on the len and call vstr_init(vstr, 0), which would then
round this up and allocate a small amount of memory for the vstr.  The
bytes constructor would then attempt to zero out all this memory, thinking
it had allocated 2^32-1 bytes.

tests/basics/bytes.py

@@ -56,3 +56,9 @@ print(x[0], x[1], x[2], x[3])
 print(bytes([128, 255]))
 # For sequence of unknown len
 print(bytes(iter([128, 255])))
+
+# Shouldn't be able to make bytes with negative length
+try:
+    bytes(-1)
+except ValueError:
+    print('ValueError')