soft_uart: check n_bits against 0 in osmo_soft_uart_tx_ubits()

Currently calling this function with n_ubits == 0 would result in
requesting one character from the application (via the .tx_cb()),
but not actually transmitting anything.  Make it return early.

Change-Id: Icbf99a9f2f6fa64dd71a5f37922f9001577c6c97
Related: OS#4396
---
M src/core/soft_uart.c
1 file changed, 18 insertions(+), 0 deletions(-)

diff --git a/src/core/soft_uart.c b/src/core/soft_uart.c
index e0f0780..4fffaee 100644
--- a/src/core/soft_uart.c
+++ b/src/core/soft_uart.c
@@ -24,6 +24,7 @@
 #include <stdint.h>
 #include <errno.h>
 
+#include <osmocom/core/utils.h>
 #include <osmocom/core/timer.h>
 #include <osmocom/core/soft_uart.h>
 
@@ -286,6 +287,9 @@
 	size_t n_frame_bits, n_chars;
 	struct msgb *msg = NULL;
 
+	if (OSMO_UNLIKELY(n_ubits == 0))
+		return -EINVAL;
+
 	/* calculate UART frame size for the effective config */
 	n_frame_bits = 1 + cfg->num_data_bits + cfg->num_stop_bits;
 	if (cfg->parity_mode != OSMO_SUART_PARITY_NONE)

