Import scanner performs bound check after buffer access

[max-hoffman]

The import scanner has a for loop that panics because we check to extend the buffer only after accessing. The check needs to precede buffer access.

panic: runtime error: index out of range [16384] with length 16384 goroutine 1 [running]: github.com/dolthub/dolt/go/cmd/dolt/commands.(*streamScanner).Scan(0xc004b99158) github.com/dolthub/dolt/go/cmd/dolt/commands/sql_statement_scanner.go:99 +0x299 

This is a simple query that repros: strings.Repeat(" ", 4096) + "insert into foo values (1,2,3)". The whitespace truncator has a bad bounds check on the 4096th byte.