Silence warnings

Silence some clang warnings:

```
de -maes -fno-strict-aliasing -fno-stack-protector -O3   -DVERSION=\"0.36\" -DXS_VERSION=\"0.36\"  "-I/Users/jacques/perl/lib/darwin/CORE"   Clone.c
Clone.xs:48:15: warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
  while (next = hv_iternext (self))
         ~~~~~^~~~~~~~~~~~~~~~~~~~
Clone.xs:48:15: note: place parentheses around the assignment to silence this warning
  while (next = hv_iternext (self))
              ^
         (                        )
Clone.xs:48:15: note: use '==' to turn this assignment into an equality comparison
  while (next = hv_iternext (self))
              ^
              ==
Clone.xs:281:15: warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
      if ( mg = mg_find(clone, 'r') )
           ~~~^~~~~~~~~~~~~~~~~~~~~
Clone.xs:281:15: note: place parentheses around the assignment to silence this warning
      if ( mg = mg_find(clone, 'r') )
              ^
           (                       )
Clone.xs:281:15: note: use '==' to turn this assignment into an equality comparison
      if ( mg = mg_find(clone, 'r') )
              ^
              ==
2 warnings generated.
```