Avoid undefined behavior

[heifner]

Flagged during a test:

/home/kevin/ext/wire-sysio/libraries/chain/include/sysio/chain/wasm_sysio_injection.hpp:155:85: runtime error: member access within address 0x5ac3c5c62480 which does not point to an object of type 'wasm_ops::op_types<>::call_t' (aka 'call<sysio::chain::wasm_ops::nop_mutator>')
0x5ac3c5c62480: note: object is of type 'sysio::chain::wasm_ops::call<sysio::chain::wasm_injections::fix_call_index>'
 00 00 00 00  08 c7 39 9c c3 5a 00 00  10 00 00 00 32 00 00 00  00 00 00 00 00 00 00 00  21 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'sysio::chain::wasm_ops::call<sysio::chain::wasm_injections::fix_call_index>'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/kevin/ext/wire-sysio/libraries/chain/include/sysio/chain/wasm_sysio_injection.hpp:155:85