enip: fix compile warnings in gcc-8

enip: fix compile warnings in gcc-8

In file included from suricata-common.h:471,
                 from app-layer-enip-common.c:27:
app-layer-enip-common.c: In function ‘DecodeCIPRequestPathPDU’:
util-debug.h:222:31: warning: ‘req_path_class8’ may be used uninitialized in this function [-Wmaybe-uninitialized]
             int _sc_log_ret = snprintf(_sc_log_msg, SC_LOG_MAX_LOG_MSG_LEN, __VA_ARGS__);   \
                               ^~~~~~~~
app-layer-enip-common.c:589:13: note: ‘req_path_class8’ was declared here
     uint8_t req_path_class8;
             ^~~~~~~~~~~~~~~
app-layer-enip-common.c:607:9: warning: ‘segment’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         switch (segment)
         ^~~~~~
app-layer-enip-common.c: In function ‘DecodeCIPResponsePDU’:
app-layer-enip-common.c:773:13: warning: ‘service’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     service &= 0x7f; //strip off top bit to get service code.  Responses have first bit as 1
             ^~
app-layer-enip-common.c: In function ‘DecodeCIPRequestPDU’:
app-layer-enip-common.c:503:25: warning: ‘path_size’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     offset += path_size * sizeof(uint16_t); //move offset past pathsize
               ~~~~~~~~~~^~~~~~~~~~~~~~~~~~
app-layer-enip-common.c:506:5: warning: ‘service’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     switch (service)
     ^~~~~~

Bug #3139.