Fix module_realloc with NULL ptr issue (#1175)

Fix module_realloc with NULL ptr issue reported by #1173.

core/iwasm/aot/aot_runtime.c

@@ -1837,7 +1837,7 @@ aot_module_realloc(AOTModuleInstance *module_inst, uint32 ptr, uint32 size,
     if (memory_inst->heap_handle.ptr) {
         addr = mem_allocator_realloc(
             memory_inst->heap_handle.ptr,
-            (uint8 *)memory_inst->memory_data.ptr + ptr, size);
+            ptr ? (uint8 *)memory_inst->memory_data.ptr + ptr : NULL, size);
     }
 
     /* Only support realloc in WAMR's app heap */

core/iwasm/interpreter/wasm_runtime.c

@@ -1917,8 +1917,8 @@ wasm_module_realloc(WASMModuleInstance *module_inst, uint32 ptr, uint32 size,
     }
 
     if (memory->heap_handle) {
-        addr = mem_allocator_realloc(memory->heap_handle,
-                                     memory->memory_data + ptr, size);
+        addr = mem_allocator_realloc(
+            memory->heap_handle, ptr ? memory->memory_data + ptr : NULL, size);
     }
 
     /* Only support realloc in WAMR's app heap */