Fix connection leak when requested object file does not exist on disk

[jefferis]

If an RDS format filehash is created on disk and an object that does not exist
is requested then there is a connection leak. If this process is repeated >124
times then R runs out of connections (which is of course bad news).

One safe approach appears to be:

• first create connection
• then open it for reading (potentially resulting in error condition)
• always close it (destroying connection) even when erroring out

The problem can be demonstrated as follows:

dbCreate("mydbRDS", "RDS")
db <- dbInit("mydbRDS", "RDS")
dbInsert(db, "a", 1:10)
showConnections(all=T)
dbFetch("a")
showConnections(all=T)
dbFetch(db, "nonexistentkey")
showConnections(all=T)
dbUnlink(db)

And the exact origin can be seen in these two snippets

# Leaks a connection
showConnections(all=TRUE)
con <- tryCatch({
    gzfile(tempfile(),'rb')
}, condition = function(cond) {
    cond
})
showConnections(all=TRUE)

# No connection leak
showConnections(all=TRUE)
val <- tryCatch({
    con<-gzfile(tempfile())
    open(con,'rb')
    readLines(con)
}, condition = function(cond) {
    cond
}, finally = {
    close(con)
})
showConnections(all=TRUE)

[jefferis]

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