AMPTInterface: Fix used uninitialized warnings

[smuzaffar]

Fixes https://github.com/cms-sw/cmssw/issues/44913 Fixes used uninitialized variable warnings reported by enabling -O3 GEANT4 IBs and GCC13 #44913)

src/GeneratorInterface/AMPTInterface/src/hipyset1.35.f:11352:72:

11352 |             ETARE=ETARE-3.*EJ*VJ*(FXYRE-0.25*F1RE)
      |                                                                        ^
  Warning: 'vj' may be used uninitialized [-Wmaybe-uninitialized]
 src/GeneratorInterface/AMPTInterface/src/hipyset1.35.f:11297:14:

11297 |             VJ=AJ-4.*EJ*XW
      |              ^
note: 'vj' was declared here
src/GeneratorInterface/AMPTInterface/src/hipyset1.35.f:11276:72:

11276 |             ETARE=ETARE+0.5*3.*EJ**2*EPS*(1.+(EPS-1.)*PHIRE)
      |                                                                        ^
  Warning: 'ej' may be used uninitialized [-Wmaybe-uninitialized]
 src/GeneratorInterface/AMPTInterface/src/hipyset1.35.f:11253:14:

11253 |             EJ=KCHG(J,1)/3.
      |              ^
note: 'ej' was declared here