Fix some -fsanitize=undefined behavior for #1475

Also fix issue with os/clock and default values that were incorrect.
The api shold have been a little nicer here to prevent this issue.

src/core/bytecode.c

@@ -140,7 +140,7 @@ void janet_bytecode_remove_noops(JanetFuncDef *def) {
                 /* relative pc is in DS field of instruction */
                 old_jump_target = i + (((int32_t)instr) >> 8);
                 new_jump_target = pc_map[old_jump_target];
-                instr += (new_jump_target - old_jump_target + (i - j)) << 8;
+                instr += (uint32_t)(new_jump_target - old_jump_target + (i - j)) << 8;
                 break;
             case JOP_JUMP_IF:
             case JOP_JUMP_IF_NIL:
@@ -149,7 +149,7 @@ void janet_bytecode_remove_noops(JanetFuncDef *def) {
                 /* relative pc is in ES field of instruction */
                 old_jump_target = i + (((int32_t)instr) >> 16);
                 new_jump_target = pc_map[old_jump_target];
-                instr += (new_jump_target - old_jump_target + (i - j)) << 16;
+                instr += (uint32_t)(new_jump_target - old_jump_target + (i - j)) << 16;
                 break;
             default:
                 break;

src/core/math.c

@@ -85,10 +85,10 @@ void janet_rng_longseed(JanetRNG *rng, const uint8_t *bytes, int32_t len) {
     uint8_t state[16] = {0};
     for (int32_t i = 0; i < len; i++)
         state[i & 0xF] ^= bytes[i];
-    rng->a = state[0] + (state[1] << 8) + (state[2] << 16) + (state[3] << 24);
-    rng->b = state[4] + (state[5] << 8) + (state[6] << 16) + (state[7] << 24);
-    rng->c = state[8] + (state[9] << 8) + (state[10] << 16) + (state[11] << 24);
-    rng->d = state[12] + (state[13] << 8) + (state[14] << 16) + (state[15] << 24);
+    rng->a = state[0] + ((uint32_t) state[1] << 8) + ((uint32_t) state[2] << 16) + ((uint32_t) state[3] << 24);
+    rng->b = state[4] + ((uint32_t) state[5] << 8) + ((uint32_t) state[6] << 16) + ((uint32_t) state[7] << 24);
+    rng->c = state[8] + ((uint32_t) state[9] << 8) + ((uint32_t) state[10] << 16) + ((uint32_t) state[11] << 24);
+    rng->d = state[12] + ((uint32_t) state[13] << 8) + ((uint32_t) state[14] << 16) + ((uint32_t) state[15] << 24);
     rng->counter = 0u;
     /* a, b, c, d can't all be 0 */
     if (rng->a == 0) rng->a = 1u;

src/core/os.c

@@ -1582,8 +1582,8 @@ JANET_CORE_FN(os_clock,
     janet_sandbox_assert(JANET_SANDBOX_HRTIME);
     janet_arity(argc, 0, 2);
 
-    JanetKeyword sourcestr = janet_optkeyword(argv, argc, 0, (const uint8_t *) "realtime");
-    if (janet_cstrcmp(sourcestr, "realtime") == 0) {
+    JanetKeyword sourcestr = janet_optkeyword(argv, argc, 0, NULL);
+    if (sourcestr == NULL || janet_cstrcmp(sourcestr, "realtime") == 0) {
         source = JANET_TIME_REALTIME;
     } else if (janet_cstrcmp(sourcestr, "monotonic") == 0) {
         source = JANET_TIME_MONOTONIC;
@@ -1596,8 +1596,8 @@ JANET_CORE_FN(os_clock,
     struct timespec tv;
     if (janet_gettime(&tv, source)) janet_panic("could not get time");
 
-    JanetKeyword formatstr = janet_optkeyword(argv, argc, 1, (const uint8_t *) "double");
-    if (janet_cstrcmp(formatstr, "double") == 0) {
+    JanetKeyword formatstr = janet_optkeyword(argv, argc, 1, NULL);
+    if (formatstr == NULL || janet_cstrcmp(formatstr, "double") == 0) {
         double dtime = (double)(tv.tv_sec + (tv.tv_nsec / 1E9));
         return janet_wrap_number(dtime);
     } else if (janet_cstrcmp(formatstr, "int") == 0) {