nwfilter: Fix memory leak and error path

Found by Coverity. If virNWFilterHashTablePut, then the 3rd arg @val
must be free'd since it would be leaked.

This also fixes potential problem on the error path where the caller
could assume the virNWFilterHashTablePut was successful when in fact
it failed leading to other issues.

src/nwfilter/nwfilter_gentech_driver.c

@@ -525,9 +525,12 @@ virNWFilterDetermineMissingVarsRec(virNWFilterDefPtr filter,
                     }
 
                     varAccess = virBufferContentAndReset(&buf);
-                    virNWFilterHashTablePut(missing_vars, varAccess,
-                                            val);
+                    rc = virNWFilterHashTablePut(missing_vars, varAccess, val);
                     VIR_FREE(varAccess);
+                    if (rc < 0) {
+                        virNWFilterVarValueFree(val);
+                        return -1;
+                    }
                 }
             }
         } else if (inc) {