nedmalloc: Fix a compile warning (exposed as error) with GCC 4.7.2

On MinGW, GCC 4.7.2 complains about

    operation on 'p->m[end]' may be undefined

Fix this by replacing the faulty lines with those of 69825ca from

    https://github.com/ned14/nedmalloc/blob/master/nedmalloc.c

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/nedmalloc/nedmalloc.c

@@ -603,7 +603,10 @@ static NOINLINE mstate FindMSpace(nedpool *p, threadcache *tc, int *lastUsed, si
 		}
 		/* We really want to make sure this goes into memory now but we
 		have to be careful of breaking aliasing rules, so write it twice */
-		*((volatile struct malloc_state **) &p->m[end])=p->m[end]=temp;
+		{
+			volatile struct malloc_state **_m=(volatile struct malloc_state **) &p->m[end];
+			*_m=(p->m[end]=temp);
+		}
 		ACQUIRE_LOCK(&p->m[end]->mutex);
 		/*printf("Created mspace idx %d\n", end);*/
 		RELEASE_LOCK(&p->mutex);