Fix OOB write in build_read_multi_rsp of gatt_sr.cc

build_read_multi_rsp is missing a bounds check, which can lead to an
OOB write when the mtu parameter is set to zero.

Add that bounds check.

Bug: 323850943
Test: atest GattSrTest
Test: researcher POC
Tag: #security
Flag: EXEMPT trivial validity checks
Ignore-AOSP-First: Security
Change-Id: I18e4325dbc9d6814220332288c85b114d0415c2f