[PATCH] Add a few suppression rules for Valgrind

Hello hackers,

I decided to run the code from master branch under Valgrind and
discovered that it reports some errors.

There are multiple reports like this one (seems to be a false alarm):

```
Invalid read of size 16
at 0x605F488: __wcsnlen_sse4_1 (in /usr/lib/libc-2.26.so)
by 0x604F5C2: wcsrtombs (in /usr/lib/libc-2.26.so)
by 0x5FE4C41: wcstombs (in /usr/lib/libc-2.26.so)
by 0x6FFAFC: wchar2char (pg_locale.c:1641)
by 0x6937A0: str_tolower (formatting.c:1599)
by 0x6F88E8: lower (oracle_compat.c:50)
by 0x43097D: ExecInterpExpr (execExprInterp.c:678)
by 0x48B201: ExecEvalExpr (executor.h:286)
by 0x48BBD6: tfuncInitialize (nodeTableFuncscan.c:369)
by 0x48B91E: tfuncFetchRows (nodeTableFuncscan.c:299)
by 0x48B245: TableFuncNext (nodeTableFuncscan.c:65)
by 0x4462E6: ExecScanFetch (execScan.c:95)
Address 0x513adab0 is 7,808 bytes inside a recently re-allocated
block of size 16,384 alloc'd
at 0x4C2CEDF: malloc (vg_replace_malloc.c:299)
by 0x7E2807: AllocSetAlloc (aset.c:934)
by 0x7EF481: palloc (mcxt.c:848)
by 0x42D2A7: ExprEvalPushStep (execExpr.c:2131)
by 0x42D7DB: ExecPushExprSlots (execExpr.c:2286)
by 0x42D723: ExecInitExprSlots (execExpr.c:2265)
by 0x429B77: ExecBuildProjectionInfo (execExpr.c:370)
by 0x44A39B: ExecAssignProjectionInfo (execUtils.c:457)
by 0x4733FE: ExecInitNestLoop (nodeNestloop.c:308)
by 0x4442F0: ExecInitNode (execProcnode.c:290)
by 0x43C5EB: InitPlan (execMain.c:1043)
by 0x43B31C: standard_ExecutorStart (execMain.c:262)
```

Also there are many reports like this one (definitely false alarm):

```
Syscall param epoll_pwait(sigmask) points to unaddressable byte(s)
at 0x60A2F90: epoll_pwait (in /usr/lib/libc-2.26.so)
by 0x5F7B24: WaitEventSetWaitBlock (latch.c:1048)
by 0x5F79E8: WaitEventSetWait (latch.c:1000)
by 0x493A79: secure_read (be-secure.c:170)
by 0x4A3DE2: pq_recvbuf (pqcomm.c:963)
by 0x4A3EAA: pq_getbyte (pqcomm.c:1006)
by 0x627AD0: SocketBackend (postgres.c:339)
by 0x628032: ReadCommand (postgres.c:512)
by 0x62D243: PostgresMain (postgres.c:4086)
by 0x581240: BackendRun (postmaster.c:4412)
by 0x5808BE: BackendStartup (postmaster.c:4084)
by 0x57CA20: ServerLoop (postmaster.c:1757)
Address 0x0 is not stack'd, malloc'd or (recently) free'd
```

I suggest a patch that adds corresponding suppression rules to the
src/tools/valgrind.supp file. Though I'm having difficulties
understanding why Valgrind complains on wcstombs and thus I can't
explain why in fact everything is OK (or it's actually not?). Hopefully
someone in the mailing list can explain this behavior. For now I left a
TODO mark in the comment.

TWIMC the full report can be found here:

https://afiskon.ru/s/3c/736207c9d2_valgrind-2018-02-20.tgz

--
Best regards,
Aleksander Alekseev