Enhance type checking for function types in loader and improve error handling (#4294)

Especially when GC is enabled, a valid item of `module->types` needs additional
checks before casting to WASMFuncType.

Also, avoid overflowing if reftype_map_count is 0.

Additionally, correctly set IN_OSS_FUZZ based on CFLAGS_ENV for sanitizer
configuration. Update ASan and UBSan messages for clarity in non-oss-fuzz
environments.

core/iwasm/interpreter/wasm.h

@@ -1243,7 +1243,7 @@ wasm_value_type_size_internal(uint8 value_type, uint8 pointer_size)
         return sizeof(int16);
 #endif
     else {
-        bh_assert(0);
+        bh_assert(0 && "Unknown value type. It should be handled ahead.");
     }
 #if WASM_ENABLE_GC == 0
     (void)pointer_size;