Fix crash when copying a map with label leaders

When copying a map (using `msCopyMap`) that contains leaders, new leaders are initialized using `msSmallAlloc` but the memory is left uninitialized.  This results in `numstyles` not being set to 0, and `msCopyLabelLeader` uses `numstyles` to free existing styles before copying styles from `src` to `dst`.  This causes a segfault.