Don't overrun in error prints

vsprintf returns the amount it would have written if the buffer had been
big enough, but we used that value directly when outputting, thus
overrunning memory and dumping stack contents.

Indicate truncation by inserting an ellipsis and newline. Slightly
increase the buffer size, so that we don't slightly decrease the maximum
printable characters because of the ellipsis insertion.

Partially addresses https://github.com/ARMmbed/mbed-os/issues/6850 by
forcing a newline when truncation happens - often truncation will drop a
newline and prevent a flush.

platform/mbed_board.c

@@ -56,25 +56,31 @@ void mbed_error_printf(const char *format, ...)
 
 void mbed_error_vprintf(const char *format, va_list arg)
 {
-#define ERROR_BUF_SIZE      (128)
     core_util_critical_section_enter();
-    char buffer[ERROR_BUF_SIZE];
-    int size = vsnprintf(buffer, ERROR_BUF_SIZE, format, arg);
-    if (size > 0) {
-#if MBED_CONF_PLATFORM_STDIO_CONVERT_NEWLINES || MBED_CONF_PLATFORM_STDIO_CONVERT_TTY_NEWLINES
-        char stdio_out_prev = '\0';
-        for (int i = 0; i < size; i++) {
-            if (buffer[i] == '\n' && stdio_out_prev != '\r') {
-                const char cr = '\r';
-                write(STDERR_FILENO, &cr, 1);
-            }
-            write(STDERR_FILENO, &buffer[i], 1);
-            stdio_out_prev = buffer[i];
-        }
-#else
-        write(STDERR_FILENO, buffer, size);
-#endif
+    char buffer[132];
+    int size = vsnprintf(buffer, sizeof buffer, format, arg);
+    if (size >= sizeof buffer) {
+        /* Output was truncated - indicate by overwriting last 4 bytes of buffer
+         * with ellipsis and newline.
+         * (Note that although vsnprintf always leaves a NUL terminator, we
+         * don't need a terminator and can use the entire buffer)
+         */
+        memcpy(&buffer[sizeof buffer - 4], "...\n", 4);
+        size = sizeof buffer;
     }
+#if MBED_CONF_PLATFORM_STDIO_CONVERT_NEWLINES || MBED_CONF_PLATFORM_STDIO_CONVERT_TTY_NEWLINES
+    char stdio_out_prev = '\0';
+    for (int i = 0; i < size; i++) {
+        if (buffer[i] == '\n' && stdio_out_prev != '\r') {
+            const char cr = '\r';
+            write(STDERR_FILENO, &cr, 1);
+        }
+        write(STDERR_FILENO, &buffer[i], 1);
+        stdio_out_prev = buffer[i];
+    }
+#else
+    write(STDERR_FILENO, buffer, size);
+#endif
     core_util_critical_section_exit();
 }