mpidummy: proper size in snprintf
mpidummy: proper size in snprintf

Fix buffer size and warning
  "could not gather data" directive output truncated writing
  22 bytes into a region of size 5 [-Wformat-truncation]

JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2