Before the variable 'bits' was initialized with 0 (commit
3470cd86), the following bug was
possible.

A function call with an empty bitmap leads to undefined
behavior. Because if 'bitmap->map_len == 0' 'unusedBits' will be <= 0
and 'sz == 1'. So the non global and non static variable 'bits' would
have never been set. Consequently the check 'bits == 0' results in
undefined behavior.

This patch clarifies the current version of the function by handling the
empty bitmap explicitly. Also, for an empty bitmap there is obviously no
bit set so we can just return -1 (indicating no bit set) right away. The
explicit check for 'bits == 0' after the loop is unnecessary because we
only get to this point if no set bit was found.
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: NBjoern Walk <bwalk@linux.vnet.ibm.com>
Signed-off-by: NMarc Hartmayer <mhartmay@linux.vnet.ibm.com>
(cherry picked from commit 7cd01a24)