qemu_capabilities: Resolve Coverity NULL_RETURNS

Adjust the initialization of qemuCaps() to check for a NULL before
attempting to dereference like other callers/users do.

src/qemu/qemu_capabilities.c

@@ -3349,10 +3349,13 @@ virQEMUCapsPtr virQEMUCapsNewForBinary(const char *binary,
                                        uid_t runUid,
                                        gid_t runGid)
 {
-    virQEMUCapsPtr qemuCaps = virQEMUCapsNew();
+    virQEMUCapsPtr qemuCaps;
     struct stat sb;
     int rv;
 
+    if (!(qemuCaps = virQEMUCapsNew()))
+        goto error;
+
     if (VIR_STRDUP(qemuCaps->binary, binary) < 0)
         goto error;