fix -Wdeclaration-after-statement for ISO C90

Tested ok on all major perl versions:
$ perlall -mq maketest
$ grep Result: log.test-linuxmint1-5.*
log.test-linuxmint1-5.10.1d:Result: PASS
log.test-linuxmint1-5.10.1-nt:Result: PASS
log.test-linuxmint1-5.12.5:Result: PASS
log.test-linuxmint1-5.12.5-nt:Result: PASS
log.test-linuxmint1-5.14.4:Result: PASS
log.test-linuxmint1-5.14.4-nt:Result: PASS
log.test-linuxmint1-5.16.3:Result: PASS
log.test-linuxmint1-5.16.3-nt:Result: PASS
log.test-linuxmint1-5.18.2:Result: PASS
log.test-linuxmint1-5.18.2-nt:Result: PASS
log.test-linuxmint1-5.8.4d-nt:Result: PASS
log.test-linuxmint1-5.8.5-nt:Result: PASS
log.test-linuxmint1-5.8.8d-nt:Result: PASS
log.test-linuxmint1-5.8.9:Result: PASS
log.test-linuxmint1-5.8.9-nt:Result: PASS

In particular it is safe not to use a heap copy of the PVX
since it's a SVp_POK with LEN=0, which is never freed, thus freed
along the shared library.