Fix return type in wasm_loader_get_custom_section (#2794)

Should return NULL instead of false.

core/iwasm/interpreter/wasm_loader.c

@@ -7026,7 +7026,7 @@ wasm_loader_get_custom_section(WASMModule *module, const char *name,
         section = section->next;
     }
 
-    return false;
+    return NULL;
 }
 #endif