Mosaic: reject game descriptions containing bad characters

Only numbers and lower-case letters are allowed.  Without this
restriction, a buffer overrun is possible.

To demonstrate the problem, load this save file into a build of Mosaic
with AddressSanitizer:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
VERSION :1:1
GAME    :6:Mosaic
PARAMS  :7:8x8a0h1
CPARAMS :7:8x8a0h1
DESC    :41:b2c3b~~2a5c6e3a55c6a5a4244e0c3a64d4b4232b
NSTATES :1:1
STATEPOS:1:1

mosaic.c

@@ -840,7 +840,8 @@ static const char *validate_desc(const game_params *params,
     while (*curr_desc != '\0') {
         if (*curr_desc >= 'a' && *curr_desc <= 'z') {
             length += *curr_desc - 'a';
-        }
+        } else if (*curr_desc < '0' || *curr_desc >= '9')
+            return "Invalid character in game description";
         length++;
         curr_desc++;
     }