util: alloc: Introduce freeing helpers that clear the memory before freeing

For a few cases where we handle secret information it's good to clear
the buffers containing sensitive data before freeing them.

Introduce VIR_DISPOSE, VIR_DISPOSE_N and VIR_DISPOSE_STRING that allow
simple clearing fo the buffers holding sensitive information on cleanup
paths.

tests/viralloctest.c

@@ -383,6 +383,41 @@ testInsertArray(const void *opaque ATTRIBUTE_UNUSED)
 }
 
 
+static int
+testDispose(const void *opaque ATTRIBUTE_UNUSED)
+{
+    int *num = NULL;
+    int *nums = NULL;
+    size_t nnums = 0;
+    char *str = NULL;
+
+    VIR_DISPOSE(num);
+    VIR_DISPOSE_N(nums, nnums);
+    VIR_DISPOSE_STRING(str);
+
+    nnums = 10;
+    VIR_DISPOSE_N(nums, nnums);
+
+    if (VIR_ALLOC(num) < 0)
+        return -1;
+
+    VIR_DISPOSE(num);
+
+    nnums = 10;
+    if (VIR_ALLOC_N(nums, nnums) < 0)
+        return -1;
+
+    VIR_DISPOSE_N(nums, nnums);
+
+    if (VIR_STRDUP(str, "test") < 0)
+        return -1;
+
+    VIR_DISPOSE_STRING(str);
+
+    return 0;
+}
+
+
 static int
 mymain(void)
 {
@@ -400,6 +435,8 @@ mymain(void)
         ret = -1;
     if (virtTestRun("insert array", testInsertArray, NULL) < 0)
         ret = -1;
+    if (virtTestRun("dispose tests", testDispose, NULL) < 0)
+        ret = -1;
 
     return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }