Fix buffer overruns and endianness issues in decoder code.

[bzbarsky-apple]

There are two issues being fixed here:

1. The code was checking that it had at least one more byte available
   to read, but then reading multiple bytes in some cases.

2. The code was reading the data (which is little-endian) via memcpy,
   which would not do the right thing on big-endian systems.

The former problem is fixed by checking that we can read the number of
bytes we plan to read; the latter problem is fixed by using
ChipEncoding to read the data explicitly as little-endian data.

fixes #1632

[rwalker-apple]

lgtm

[github-actions]

Size increase report for "nrfconnect-example-build"

File	Section	File	VM
chip-nrf52840-lock-example.elf	text	44	44
chip-nrf52840-lock-example.elf	[LOAD #1 [RWX]]	4	4
chip-nrf52840-lock-example.elf	rodata	-64	-64

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-nrf52840-lock-example.elf and ./pull_artifact/chip-nrf52840-lock-example.elf:

sections,vmsize,filesize
.debug_info,0,902
.debug_frame,0,516
.debug_line,0,469
.debug_abbrev,0,256
.debug_aranges,0,112
.debug_ranges,0,112
.symtab,0,64
.debug_str,0,59
.strtab,0,54
text,44,44
[LOAD #1 [RWX]],4,4
[Unmapped],0,-16
rodata,-64,-64

[github-actions]

Size increase report for "esp32-example-build"

File	Section	File	VM
chip-wifi-echo.elf	.flash.text	-40	-40
chip-wifi-echo.elf	.flash.rodata	-64	-64

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-wifi-echo.elf and ./pull_artifact/chip-wifi-echo.elf:

sections,vmsize,filesize
.debug_info,0,895
.debug_line,0,434
.debug_frame,0,336
.debug_abbrev,0,277
.xt.lit._ZNK4chip8OptionalItE5ValueEv,0,200
.shstrtab,0,142
.debug_aranges,0,112
.debug_ranges,0,112
.symtab,0,96
.xt.prop._ZN2nl9ByteOrder18Swap16LittleToHostEt,0,80
.xt.prop._ZN6ReaderILi1EE4readERPKh,0,76
.xt.prop._ZN6ReaderILi2EE4readERPKh,0,76
[Unmapped],0,63
.debug_str,0,59
.strtab,0,54
.flash.text,-40,-40
.flash.rodata,-64,-64
.xt.lit._ZN4chip8Encoding5Read8ERPKh,0,-200

[github-actions]

Size increase report for "gn_nrf-example-build"

File	Section	File	VM

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

[github-actions]

Size increase report for "gn_linux-example-build"

File	Section	File	VM

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

[rwalker-apple]

@hawk248 @jelderton @saurabhst @andy31415 ?