hhvm: avoid a GCC7 warning by adding positive int constraints

hhvm: avoid a GCC7 warning by adding positive int constraints

Summary:
GCC 7 compile fails with:
  In member function 'void HPHP::ThriftBuffer::read(char*, int)',
    inlined from 'void HPHP::ThriftBuffer::read(int8_t&)'
      at hphp/runtime/base/thrift-buffer.h:129:11,
    inlined from 'void HPHP::ThriftBuffer::skip(int8_t)'
      at hphp/runtime/base/thrift-buffer.cpp:135:27:
  hphp/runtime/base/thrift-buffer.cpp:94:19: error:
    'void* memcpy(void*, const void*, size_t)':
    specified size between 18446744071562067968 and 18446744073709551615
    exceeds maximum object size 9223372036854775807 [-Werror=stringop-overflow=]

So for int8_t the compiler thinks that `int avail` at
hphp/runtime/base/thrift-buffer.cpp:83 may go negative.
There is no way that m_p can go beyond m_pEnd,
so use not_reached() to enforce only positive values for avail.

Reviewed By: markw65

Differential Revision: D6372117

fbshipit-source-id: 0b16e0a4b2a98c036174643f7bb83a08a7ab507d