MFV c3ccd112: Correct off-by-ones in free_exception of emergency buffer

Note, this has been broken since import in r227825.

PR:		https://github.com/pathscale/libcxxrt/issues/29
Reviewed by:	emaste (earlier version), kan (informally)
Obtained from:	Anton Rang
Relnotes:	yes
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D3733

contrib/libcxxrt/exception.cc

@@ -516,7 +516,7 @@ static void emergency_malloc_free(char *ptr)
 			break;
 		}
 	}
-	assert(buffer > 0 &&
+	assert(buffer >= 0 &&
 	       "Trying to free something that is not an emergency buffer!");
 	// emergency_malloc() is expected to return 0-initialized data.  We don't
 	// zero the buffer when allocating it, because the static buffers will
@@ -556,7 +556,7 @@ static void free_exception(char *e)
 {
 	// If this allocation is within the address range of the emergency buffer,
 	// don't call free() because it was not allocated with malloc()
-	if ((e > emergency_buffer) &&
+	if ((e >= emergency_buffer) &&
 	    (e < (emergency_buffer + sizeof(emergency_buffer))))
 	{
 		emergency_malloc_free(e);