How to reproduce:
1. Open the attached "SmallerRepro" project and "SampleScene"
2. Enter playmode

Result - Crash occurs on:

========== OUTPUTTING STACK TRACE ==================
0x00007FFB975E1D3D (mono-2.0-bdwgc) [f:\dd\vctools\crt\vcruntime\src\string\amd64\memcpy.asm:418] MoveSmall
0x00007FFB9735BD91 (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\marshal.c:11006] ves_icall_System_Runtime_InteropServices_Marshal_copy_to_unmanaged
0x0000000019E3EF84 (Mono JIT Code) (wrapper managed-to-native) System.Runtime.InteropServices.Marshal:copy_to_unmanaged (System.Array,int,intptr,int)
0x0000000019E3EE3B (Mono JIT Code) System.Runtime.InteropServices.Marshal:Copy (byte[],int,intptr,int)
0x0000000019E35B83 (Mono JIT Code) [C:\Users\TomasM\Desktop\SmallerRepro\Assets\Scripts\ObjectToJSON.cs:221] ObjectToJSON:getBase64 (UnityEngine.Sprite,int)
0x0000000019E34EAB (Mono JIT Code) [C:\Users\TomasM\Desktop\SmallerRepro\Assets\Scripts\ObjectToJSON.cs:169] ObjectToJSON:printBase64 (System.Collections.Generic.List`1<playerComponent>)
0x0000000019E29333 (Mono JIT Code) [C:\Users\TomasM\Desktop\SmallerRepro\Assets\Scripts\ObjectToJSON.cs:39] ObjectToJSON:Start ()
...

Reproducible with - 2018.2.0a5, 2018.1.0b11, 2017.3.2f1, 2017.3.0a1

By design --> string getBase64(Sprite sprite, int index) is writing into random memory. The `Marshal.Copy` call is writing to the BitMapData memory that is smaller than the bytes array that was retrieved