psMetadataIteratorAlloc produces SEGV when regex != NULL

[Paul Price]

#0 0x00432b70 in regcomp () from /lib/tls/libc.so.6
#1 0x00d78fef in psMetadataIteratorAlloc (md=0x8dd022c, location=0,

regex=0x81ae786 "^{-chip$") at psMetadata.c:924}

This is because regcomp requires a pointer to an allocated buffer, instead of NULL.

[Paul Price]

Fixed by adding psAlloc for newIter->regex, and refactoring code in
psMetadataIteratorAlloc.

No tests should break, because no tests call psMetadataIteratorAlloc with regex
!= NULL (!!!).