avoid dangling pointer when indexvalue does out of scope

[xiaoqiangwang]

The problem shows up on macOS but not on Linux and Windows as I checked.

With caProvider, the value will not change.

% pvput -p ca catest.SCAN 1
Old : 2022-02-19 15:25:35.846  MINOR DEVICE HIGH (0) Passive
New : 2022-02-19 15:25:35.846  MINOR DEVICE HIGH (0) Passive

With pvaProvider, the value is changed.

% pvput -p pva catest.SCAN 1
Old : 2022-02-19 15:25:35.846  MINOR DEVICE HIGH (0) Passive
New : 2022-02-19 15:25:35.846  MINOR DEVICE HIGH (1) Event

[AppVeyorBot]

:white_check_mark: Build pvAccessCPP 1.0.59 completed (commit https://github.com/epics-base/pvAccessCPP/commit/a4e10ffaf8 by @xiaoqiangwang)

[anjohnson]

I can confirm this fixes pvput to a SCAN field using the caProvider on MacOS, thanks!

Unfortunately adding a test case to the testCa/testCaProvider.cpp code to detect this doesn't seem trivial to do, the test output showed that something was wrong without this fix when I made this change:

diff --git a/testCa/testCaProvider.cpp b/testCa/testCaProvider.cpp
index 30ce61ce..8703239f 100644
--- a/testCa/testCaProvider.cpp
+++ b/testCa/testCaProvider.cpp
@@ -833,6 +833,7 @@ void testClientTypes(void)
     checkClient("DBRdoubleArray", "1 2 3");
     checkClient("DBRstringArray", "aa bb cc");
     checkClient("DBRmbbout", "2");
+    checkClient("DBRmbbout.SCAN", "4");
     checkClient("DBRbinaryout", "1");
 }

@@ -855,7 +856,7 @@ MAIN(testCaProvider)
 #if HAS_DBUNITTEST
         // Set TESTCAP_USE_SYSTEM in environment to use other impl.
         if (!getenv("TESTCAP_USE_SYSTEM")) {
-            testPlan(143);
+            testPlan(149);
             TEST_CLIENT_TYPES(TestIocUnit)
         } else
 #endif

but it didn't cause any of the actual tests on that channel to fail:

# TestClient::put DBRmbbout.SCAN := 4
 filename="(null)" line number=0PV: DBRmbbout scanAdd detected illegal SCAN value

I am about to merge this PR (without the above test modifications).

I don't have time to rewrite testCaProvider.cpp at the moment, I think it probably needs that level of work to fix properly.