rtos/mqx: Fix uninitialized parts of symbol table

rtos/mqx: Fix uninitialized parts of symbol table

Memory for the symbol table was allocated by malloc but not initialized other
than with the symbol name. Therefore `address` and `optional` members were
having arbitrary values leading to every symbol being optional most of the
time which messes up RTOS auto-detection. Memory will now be zero-initialized
as in other RTOS implementations.

Change-Id: I6c6e31ec1ef7e043061adf8c695b2139620e005d
Signed-off-by: Daniel Krebs <github@daniel-krebs.net>
Reviewed-on: http://openocd.zylin.com/3017
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>