scanner: allow for a zero terminated string as keymap

As the documentation for xkb_keymap_new_from_buffer() states, the "input string
does not have to be zero-terminated". The actual implementation however failed
with "unrecognized token/syntax error" when it encountered a null byte.

Fix this by allowing a null byte at the last position of the buffer. Anything
else is likely a client error anyway.

Fixes #307

src/keymap.c

@@ -194,6 +194,10 @@ xkb_keymap_new_from_buffer(struct xkb_context *ctx,
     if (!keymap)
         return NULL;
 
+    /* Allow a zero-terminated string as a buffer */
+    if (length > 0 && buffer[length - 1] == '\0')
+        length--;
+
     if (!ops->keymap_new_from_string(keymap, buffer, length)) {
         xkb_keymap_unref(keymap);
         return NULL;

test/buffercomp.c

@@ -46,7 +46,9 @@ main(int argc, char *argv[])
     original = test_read_file(DATA_PATH);
     assert(original);
 
-    keymap = test_compile_buffer(ctx, original, strlen(original));
+    /* Load a prebuild keymap, once without, once with the trailing \0 */
+    for (int i = 0; i <= 1; i++) {
+        keymap = test_compile_buffer(ctx, original, strlen(original) + i);
         assert(keymap);
 
         dump = xkb_keymap_get_as_string(keymap, XKB_KEYMAP_USE_ORIGINAL_FORMAT);
@@ -66,9 +68,11 @@ main(int argc, char *argv[])
             assert(0);
         }
 
-    free(original);
         free(dump);
         xkb_keymap_unref(keymap);
+    }
+
+    free(original);
 
     /* Make sure we can't (falsely claim to) compile an empty string. */
     keymap = test_compile_buffer(ctx, "", 0);