avutil/bprint: Allow size == 0 in av_bprint_init_for_buffer()

The AVBPrint API guarantees that the string buffer is always
zero-terminated; in order to honour this guarantee, there
obviously must be a string buffer at all and it must have
a size >= 1. Therefore av_bprint_init_for_buffer() treats
passing a NULL buffer or size == 0 as invalid data that
leads to undefined behaviour, namely NPD in case NULL is provided
or a write to a buffer of size 0 in case size == 0.

But it would be easy to support this, namely by using the internal
buffer with AV_BPRINT_SIZE_COUNT_ONLY in case size == 0.

There is a reason to allow this: Several functions like
av_channel_(description|name) are actually wrappers
around corresponding AVBPrint functions. They accept user
provided buffers and are supposed to return the required
size of the buffer, which would allow the user to call
it once to get the required buffer size and call it once
more after having allocated the buffer.
If av_bprint_init_for_buffer() treats size == 0 as invalid,
all these users would need to check for this themselves
and basically add the same codeblock that this patch
adds to av_bprint_init_for_buffer().

This change is in line with e.g. snprintf() which also allows
the pointer to be NULL in case size is zero.

This fixes Coverity issues #1503074, #1503076 and #1503082;
all of these issues are about providing NULL to the channel-layout
functions that are wrappers around AVBPrint versions.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/bprint.c

@@ -84,6 +84,11 @@ void av_bprint_init(AVBPrint *buf, unsigned size_init, unsigned size_max)
 
 void av_bprint_init_for_buffer(AVBPrint *buf, char *buffer, unsigned size)
 {
+    if (size == 0) {
+        av_bprint_init(buf, 0, AV_BPRINT_SIZE_COUNT_ONLY);
+        return;
+    }
+
     buf->str      = buffer;
     buf->len      = 0;
     buf->size     = size;

libavutil/bprint.h

@@ -144,6 +144,9 @@ void av_bprint_init(AVBPrint *buf, unsigned size_init, unsigned size_max);
  * Init a print buffer using a pre-existing buffer.
  *
  * The buffer will not be reallocated.
+ * In case size equals zero, the AVBPrint will be initialized to use
+ * the internal buffer as if using AV_BPRINT_SIZE_COUNT_ONLY with
+ * av_bprint_init().
  *
  * @param buf     buffer structure to init
  * @param buffer  byte buffer to use for the string data

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  58
-#define LIBAVUTIL_VERSION_MINOR  15
+#define LIBAVUTIL_VERSION_MINOR  16
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \