malloc: Fix MALLOC_DEBUG -Wundef warning

MALLOC_DEBUG is set optionally on the command line. Default the value
to zero if it is not set on the command line, and test its value
with #if rather than #ifdef. Verified the code is identical before
and after this change apart from line numbers.

ChangeLog:

2014-04-11  Will Newton  <will.newton@linaro.org>

	* malloc/malloc.c [!MALLOC_DEBUG]: #define MALLOC_DEBUG
	to zero if it is not defined elsewhere.  (mtrim): Test
	the value of MALLOC_DEBUG with #if rather than #ifdef.