Fix Memory Leak in virQEMUCapsInitGuestFromBinary()

While running qemucaps2xmltest, it was found that valgrind pointed out
the following memory leaks:

==29896== 0 bytes in 1 blocks are definitely lost in loss record 1 of 65
==29896==    at 0x4A0577B: calloc (vg_replace_malloc.c:593)
==29896==    by 0x4C6B45E: virAllocN (viralloc.c:191)
==29896==    by 0x4232A9: virQEMUCapsGetMachineTypesCaps (qemu_capabilities.c:1999)
==29896==    by 0x4234E7: virQEMUCapsInitGuestFromBinary (qemu_capabilities.c:789)
==29896==    by 0x41F10B: testQemuCapsXML (qemucaps2xmltest.c:118)
==29896==    by 0x41FFD1: virtTestRun (testutils.c:201)
==29896==    by 0x41EE7A: mymain (qemucaps2xmltest.c:203)
==29896==    by 0x42074D: virtTestMain (testutils.c:789)
==29896==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)
==29896==
==29896== 0 bytes in 1 blocks are definitely lost in loss record 2 of 65
==29896==    at 0x4A0577B: calloc (vg_replace_malloc.c:593)
==29896==    by 0x4C6B45E: virAllocN (viralloc.c:191)
==29896==    by 0x4232A9: virQEMUCapsGetMachineTypesCaps (qemu_capabilities.c:1999)
==29896==    by 0x4234E7: virQEMUCapsInitGuestFromBinary (qemu_capabilities.c:789)
==29896==    by 0x41F10B: testQemuCapsXML (qemucaps2xmltest.c:118)
==29896==    by 0x41FFD1: virtTestRun (testutils.c:201)
==29896==    by 0x41EEA3: mymain (qemucaps2xmltest.c:204)
==29896==    by 0x42074D: virtTestMain (testutils.c:789)
==29896==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)
Signed-off-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_capabilities.c

@@ -1994,12 +1994,13 @@ int virQEMUCapsGetMachineTypesCaps(virQEMUCapsPtr qemuCaps,
 {
     size_t i;
 
-    *nmachines = 0;
     *machines = NULL;
-    if (VIR_ALLOC_N(*machines, qemuCaps->nmachineTypes) < 0)
-        goto error;
     *nmachines = qemuCaps->nmachineTypes;
 
+    if (*nmachines &&
+        VIR_ALLOC_N(*machines, qemuCaps->nmachineTypes) < 0)
+        goto error;
+
     for (i = 0; i < qemuCaps->nmachineTypes; i++) {
         virCapsGuestMachinePtr mach;
         if (VIR_ALLOC(mach) < 0)