Fix issues caught by Clang's -Wpointer-bool-conversion warning

MariaDB 10.0 triggers Clang 3.6's -Wpointer-bool-conversion warnings. These warnings trigger when a pointer which the compiler knows cannot be null is converted to a bool (e.g. in a comparison). These mostly arise from statically or stack-allocated arrays or structures, but may also be triggered by _attribute_((nonnull)) on functions.

The attached patch cleans up these issues in the client, server, and InnoDB — mostly by removing or adjusting the comparison — so that they compile successfully with -Werror=pointer-bool-conversion.

Thanks,
— David