lib: circbuf: avoid possible null pointer dereference

We should not first dereference p and afterwards assert that is
was not NULL. Instead do the assert first.

The problem was indicated by cppcheck.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

lib/circbuf.c

@@ -41,11 +41,13 @@ int buf_free (circbuf_t * buf)
 int buf_pop (circbuf_t * buf, char *dest, unsigned int len)
 {
 	unsigned int i;
-	char *p = buf->top;
+	char *p;
 
 	assert (buf != NULL);
 	assert (dest != NULL);
 
+	p = buf->top;
+
 	/* Cap to number of bytes in buffer */
 	if (len > buf->size)
 		len = buf->size;
@@ -69,11 +71,13 @@ int buf_push (circbuf_t * buf, const char *src, unsigned int len)
 {
 	/* NOTE:  this function allows push to overwrite old data. */
 	unsigned int i;
-	char *p = buf->tail;
+	char *p;
 
 	assert (buf != NULL);
 	assert (src != NULL);
 
+	p = buf->tail;
+
 	for (i = 0; i < len; i++) {
 		*p++ = src[i];
 		if (p == buf->end) {