Ensure that lfields[] is fully initialized.

Ensure that lfields[] is fully initialized.

Fixes compilation error:

In file included from .../src/xbt/mmalloc/mm.c:24:0:
.../src/xbt/mmalloc/mm_diff.c: In function ‘get_addr_memory_map’:
.../src/xbt/mmalloc/mm_diff.c:373:9: error: ‘lfields[5]’ may be used uninitialized in this function [-Werror=uninitialized]
cc1: all warnings being treated as errors

In fact, once strtok returns NULL, subsequent searches with the same parameters
return NULL too.