extmod/modubinascii: Add check for empty buffer passed to hexlify.

Previous to this patch hexlify(b'', b':') would lead to a bad crash due to
the computed length of the result being -1=0xffffffff.

extmod/modubinascii.c

@@ -42,6 +42,12 @@ mp_obj_t mod_binascii_hexlify(size_t n_args, const mp_obj_t *args) {
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[0], &bufinfo, MP_BUFFER_READ);
 
+    // Code below assumes non-zero buffer length when computing size with
+    // separator, so handle the zero-length case here.
+    if (bufinfo.len == 0) {
+        return mp_const_empty_bytes;
+    }
+
     vstr_t vstr;
     size_t out_len = bufinfo.len * 2;
     if (n_args > 1) {

tests/extmod/ubinascii_micropython.py

@@ -10,3 +10,6 @@ except ImportError:
 # two arguments supported in uPy but not CPython
 a = binascii.hexlify(b'123', ':')
 print(a)
+
+# zero length buffer
+print(binascii.hexlify(b'', b':'))

tests/extmod/ubinascii_micropython.py.exp

@@ -1 +1,2 @@
 b'31:32:33'
+b''