extmod/modbinascii: Fix buffer length error.

The mod_binascii_a2b_base64() function allocates a buffer which may be
too small. It needs to be no less than three-quarters of the input
length, but is calculated as (<length> / 4) * 3 + 1, which may be less
due to integer division. Changed to (<length> * 3) / 4 + 1.

Signed-off-by: Duncan Lowther <Duncan.Lowther@glasgow.ac.uk>

extmod/modbinascii.c

@@ -71,7 +71,7 @@ STATIC mp_obj_t mod_binascii_a2b_base64(mp_obj_t data) {
     byte *in = bufinfo.buf;
 
     vstr_t vstr;
-    vstr_init(&vstr, (bufinfo.len / 4) * 3 + 1); // Potentially over-allocate
+    vstr_init(&vstr, (bufinfo.len * 3) / 4 + 1); // Potentially over-allocate
     byte *out = (byte *)vstr.buf;
 
     uint shift = 0;