Assert valid operand length (#558)

* Assert valid bytecode and operand length

EVM bytecode comes in 32-byte chunks, and contracts require at least one
segment to be created. An example, originally written by @ggrieco-tob in issue #546, is included.
Unsure what expected behavior should be, but initial state has a problem when a contract is
created with a single byte.

Also added some exception handling in `parse_operand()` to catch invalid
instructions, e.g. a PUSH1 instruction followed by no bytes.

Signed-off-by: Cole Lightfighter <cole@onicsla.bz>

* Fix SLOAD invalid memory access

Simple check to ensure address storage is in global_storage before
attempting to load data.

* Remove invalid assertions

examples/evm/experiment.py

@@ -0,0 +1,21 @@
+from seth import *
+
+seth = ManticoreEVM()
+seth.verbosity(3)
+user_account = seth.create_account(balance=1000)
+
+bytecode = '`'
+#Initialize contract
+contract_account = seth.create_contract(owner=user_account,
+                                          balance=0,
+                                          init=bytecode)
+
+def explore(state):
+    pass
+
+seth.add_hook(None, explore)
+seth.transaction(  caller=user_account,
+                    address=contract_account,
+                    value=None,
+                    data='',
+                 )