Manually preserve cconvert values until sqlite3_step (#308)

Fixes #306.

The core issue here is that when we `bind!` some values to an insert
statement, the _cconvert_ed value needs to remain valid (i.e. not
GC-able) until we execute the insert statement in `sqlite3_step`.
In each `SQLite.Stmt`, we have a `params` field for storing these
values, but we weren't accounting for the fact that some values,
(in the original issue case, InlineStrings), may be `cconvert`ed
to a different value when they're actually bound, and it's the cconverted
value that we need to store in `params` until the statement is executed.

Author: quinnj

src/SQLite.jl

@@ -140,6 +140,7 @@ DB is disconnected or when [`SQLite.finalize_statements!`](@ref) is explicitly c
 mutable struct Stmt <: DBInterface.Statement
     db::DB
     stmt_wrapper::StmtWrapper
+    # used for holding references to bound statement values via bind!
     params::Dict{Int,Any}
 
     function Stmt(db::DB, sql::AbstractString; register::Bool = true)
@@ -311,11 +312,12 @@ function bind!(stmt::Stmt, i::Integer, val::Nothing)
     @CHECK stmt.db C.sqlite3_bind_null(_get_stmt_handle(stmt), i)
 end
 function bind!(stmt::Stmt, i::Integer, val::AbstractString)
-    stmt.params[i] = val
+    cval = Base.cconvert(Ptr{Cchar}, val)
+    stmt.params[i] = cval
     @CHECK stmt.db C.sqlite3_bind_text(
         _get_stmt_handle(stmt),
         i,
-        val,
+        cval,
         sizeof(val),
         C_NULL,
     )