fix: [stix2 export] Fixed number_of_sections field in STIX PE extenstion
fix: [stix2 export] Fixed number_of_sections field in STIX PE extenstion

- Avoiding issues when the section referenced in the
  MISP pe object is actually not in the event

FloatingGhost/MISP,FloatingGhost/MISP,FIRSTdotorg/MISP,Rafiot/MISP,SteveClement/MISP,SteveClement/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,Rafiot/MISP,MISP/MISP,FloatingGhost/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP,FloatingGhost/MISP,Rafiot/MISP,Rafiot/MISP,FIRSTdotorg/MISP,MISP/MISP