coverity: Fix Sizeof not portable (SIZEOF_MISMATCH)

Coverity reports that passing argument 'sizeof (ctx->attrs)' to calloc and
then casting the return value to 'char **' looks suspicious. In fact, this
is just plain wrong. It happens to work, in this particular case since
sizeof (char ** const) happens to be equal to sizeof (char *), but this is
not a portable assumption.

So fix that by using 'sizeof (*ctx->attrs)'

Signed-off-by: Robin Getz <robin.getz@analog.com>

Author: rgetz

context.c

@@ -62,12 +62,12 @@ char * iio_context_create_xml(const struct iio_context *ctx)
 		len += sizeof(" description=\"\"") - 1;
 	}
 
-	ctx_attrs = calloc(ctx->nb_attrs, sizeof(ctx->attrs));
+	ctx_attrs = calloc(ctx->nb_attrs, sizeof(*ctx->attrs));
 	if (!ctx_attrs) {
 		errno = ENOMEM;
 		return NULL;
 	}
-	ctx_values = calloc(ctx->nb_attrs, sizeof(ctx->values));
+	ctx_values = calloc(ctx->nb_attrs, sizeof(*ctx->values));
 	if (!ctx_values) {
 		errno = ENOMEM;
 		goto err_free_ctx_attrs;