Fix xbcloud out of range memory read

[imrivera]

Fix a memory out of bounds read when chunk_name.size() < 21

- if (chunk_name.size() < 22 && chunk_name[chunk_name.size() - 21] != '.') {
+ if (chunk_name.size() < 22 || chunk_name[chunk_name.size() - 21] != '.') {

chunk_name.size()	Before (&&)	After (||)
< 21	- 1st condition is true - 2nd condition use negative index	- 1st condition is true - 2nd condition not evaluated
== 21	- 1st condition is true - 2nd condition using index 0, ok but useless if we want names longer than 21 chars	- 1st condition is true - 2nd condition not evaluated
>= 22	- 1st condition is false - 2nd condition is not evaluated	- 1st condition is false - 2nd condition is evaluated with an index always > 0

[it-percona-cla]

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