Fix an assert() in os_unix.c that is not true when using RBU.

FossilOrigin-Name: 028ed8618af012b51104a06cc155b45d98bc4ff833d4431b9c38fd6851a74e82

manifest

@@ -1,5 +1,5 @@
-C Restrict\sthe\sscope\sof\sinternal-use\sfunctions\sin\salter.c.
-D 2018-09-12T15:51:56.382
+C Fix\san\sassert()\sin\sos_unix.c\sthat\sis\snot\strue\swhen\susing\sRBU.
+D 2018-09-12T20:32:19.959
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
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@@ -485,7 +485,7 @@ F src/os.c 8aeb0b0f40f8f5b0da03fe49706695adaf42d2f516ab95abc72e86c245e119de
 F src/os.h 48388821692e87da174ea198bf96b1b2d9d83be5dfc908f673ee21fafbe0d432
 F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
 F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
-F src/os_unix.c 03a5cd0e254dfecd86e800e442a35636c755ececb9e33863a978d9ab23120938
+F src/os_unix.c d8e37bc3af9b15a52c3fecd366c95416251aa8d89481dd56170bad538b421721
 F src/os_win.c 070cdbb400097c6cda54aa005356095afdc2f3ee691d17192c54724ef146a971
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
 F src/pager.c a0d8f686ef64549ad5b356fd30429bd9ee7a06dd42b4d6faa096352ff26b1c5b
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manifest.uuid

@@ -1 +1 @@
-4396914f8e1a847d86c22fb0b91e7431cf19aa260cbec520af3564248d6897ad
+028ed8618af012b51104a06cc155b45d98bc4ff833d4431b9c38fd6851a74e82

src/os_unix.c

@@ -4879,7 +4879,9 @@ static void unixShmBarrier(
 ){
   UNUSED_PARAMETER(fd);
   sqlite3MemoryBarrier();         /* compiler-defined memory barrier */
-  assert( unixFileMutexNotheld((unixFile*)fd) );
+  assert( fd->pMethods->xLock==nolockLock 
+       || unixFileMutexNotheld((unixFile*)fd) 
+  );
   unixEnterMutex();               /* Also mutex, for redundancy */
   unixLeaveMutex();
 }