ASoC: soc-core: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200507192228.GA16355@embeddedorSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/soc-dapm.h

@@ -689,7 +689,7 @@ struct snd_soc_dapm_context {
 /* A list of widgets associated with an object, typically a snd_kcontrol */
 struct snd_soc_dapm_widget_list {
 	int num_widgets;
-	struct snd_soc_dapm_widget *widgets[0];
+	struct snd_soc_dapm_widget *widgets[];
 };
 
 #define for_each_dapm_widgets(list, i, widget)				\

include/sound/soc.h

@@ -1172,7 +1172,7 @@ struct snd_soc_pcm_runtime {
 	unsigned int fe_compr:1; /* for Dynamic PCM */
 
 	int num_components;
-	struct snd_soc_component *components[0]; /* CPU/Codec/Platform */
+	struct snd_soc_component *components[]; /* CPU/Codec/Platform */
 };
 /* see soc_new_pcm_runtime()  */
 #define asoc_rtd_to_cpu(rtd, n)   (rtd)->dais[n]