mat ex9.c: fix use of VecAXPY()
MatCompositeMerge: fix invalid access to scale after MatHeaderReplace()

We need scaling vectors to outlive destruction of the Mat_Composite.

Reported by Nachiket Gokhale <gokhalen@gmail.com> [petsc-maint #156279]
petscviewer.h: end with __FUNCT__ = "User provided function"
Merge branch 'jed/fix-mat-composite-merge-invalid-read'

* jed/fix-mat-composite-merge-invalid-read:
  MatCompositeMerge: fix invalid access to scale after MatHeaderReplace()
  mat ex9.c: fix use of VecAXPY()