keymap: fix modular arithmetic

Consider num_groups = 1.

Fixes: 67b03ce ("state: correctly wrap state->locked_group and ->group")

src/keymap-priv.c

@@ -231,15 +231,17 @@ XkbWrapGroupIntoRange(int32_t group,
 
     case RANGE_WRAP:
     default:
+        static_assert(XKB_MAX_GROUPS < INT32_MAX, "Max groups don't fit");
         /*
-         * C99 says a negative dividend in a modulo operation always
-         * gives a negative result.
+         * % returns the remainder of the division, a number in the open interval
+         * (-num_groups, num_groups). If the number is negative, we must add num_groups
+         * to get the modulus.
          */
-        if (group < 0) {
-            static_assert(XKB_MAX_GROUPS < INT32_MAX, "Max groups don't fit");
-            return ((int32_t) num_groups + (group % (int32_t) num_groups));
+        int rem = group % (int32_t) num_groups;
+        if (rem < 0) {
+            return rem + (int32_t) num_groups;
         } else {
-            return group % num_groups;
+            return rem;
         }
     }
 }