[1.0.1 -> main] Verify index >= 0 in apply_diff and fix warnings

[linh2931]

Forwards #722

index in apply_diff can be a signed int and can be negative. Currently index is only checked not greater than the size.

This also results in warnings in numerous places as apply_diff is in a header file which is included by a number of files:

libraries/libfc/include/fc/container/ordered_diff.hpp:111:35: warning: comparison of integer expressions of different signedness: ‘std::ptrdiff_t’ {aka ‘long int’} and ‘std::vector<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
...
ibraries/libfc/include/fc/container/ordered_diff.hpp:119:26: warning: comparison of integer expressions of different signedness: ‘std::tuple_element<0, std::pair<int, char> >::type’ {aka ‘int’} and ‘std::vector<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]

Resolves #723

[ericpassmore]

Note:start
category: Other
component: Internal
summary: Additional checks to harden the processing of the finalizer and proposer policy diffs from block header extension.
Note:end