blktap2: Fix two 'maybe uninitialized' variables

for which gcc 4.9.0 complains about, like this:

block-qcow.c: In function `get_cluster_offset':
block-qcow.c:431:3: error: `tmp_ptr' may be used uninitialized in this function
[-Werror=maybe-uninitialized]
   memcpy(tmp_ptr, l1_ptr, 4096);
   ^
block-qcow.c:606:7: error: `tmp_ptr2' may be used uninitialized in this
function [-Werror=maybe-uninitialized]
   if (write(s->fd, tmp_ptr2, 4096) != 4096) {
       ^
cc1: all warnings being treated as errors
/home/dario/Sources/xen/xen/xen.git/tools/blktap2/drivers/../../../tools/Rules.mk:89:
 recipe for target 'block-qcow.o' failed
make[5]: *** [block-qcow.o] Error 1

The proper behavior is to return upon allocation failure.
About what to return, 0 seems the best option, looking
at both the function and the call sites.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/blktap2/drivers/block-qcow.c

@@ -427,6 +427,7 @@ static uint64_t get_cluster_offset(struct tdqcow_state *s,
 
 		if (posix_memalign((void **)&tmp_ptr, 4096, 4096) != 0) {
 			DPRINTF("ERROR allocating memory for L1 table\n");
+                        return 0;
 		}
 		memcpy(tmp_ptr, l1_ptr, 4096);
 
@@ -600,6 +601,7 @@ found:
 		
 		if (posix_memalign((void **)&tmp_ptr2, 4096, 4096) != 0) {
 			DPRINTF("ERROR allocating memory for L1 table\n");
+                        return 0;
 		}
 		memcpy(tmp_ptr2, l2_ptr, 4096);
 		lseek(s->fd, l2_offset + (l2_sector << 12), SEEK_SET);