Correctly handle some short save files

A save file that ended in the middle of a value before the "SAVEFILE"
field had been loaded would cause a read from uninitialised memory.
While technically undefined behaviour this was practically pretty
harmless.  Fixed by handling unexpected EOF here the same an
unexpected EOF anywhere else.

This bug could be demonstrated by loading a truncated save file like
this in a build with MemorySanitizer enabled:

SAVEFILE:41:Simo

1 files changed, 2 insertions, 2 deletions

diff --git a/midend.c b/midend.c
index 2365258..90bcde2 100644
--- a/midend.c
+++ b/midend.c
@@ -2340,7 +2340,7 @@ static const char *midend_deserialise_internal(
 
         val = snewn(len+1, char);
         if (!read(rctx, val, len)) {
-            if (started)
+            /* unexpected EOF */
             goto cleanup;
         }
         val[len] = '\0';
@@ -2747,7 +2747,7 @@ const char *identify_game(char **name,
 
         val = snewn(len+1, char);
         if (!read(rctx, val, len)) {
-            if (started)
+            /* unexpected EOF */
             goto cleanup;
         }
         val[len] = '\0';