rts/LdvProfile.c: Fix NULL dereference on shutdown

Test Plan: validate

Reviewers: carter, austin, simonmar, bgamari

Reviewed By: simonmar, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2154

GHC Trac Issues: #11978

rts/LdvProfile.c

@@ -178,6 +178,9 @@ processNurseryForDead( void )
     StgPtr p;
     bdescr *bd;
 
+    if (MainCapability.r.rNursery == NULL)
+        return;
+
     for (bd = MainCapability.r.rNursery->blocks; bd != NULL; bd = bd->link) {
         p = bd->start;
         while (p < bd->free) {

testsuite/tests/profiling/should_run/T11978a.hs

+main :: IO ()
+main = putStrLn "Hello!"

testsuite/tests/profiling/should_run/T11978a.stdout

+Hello!

testsuite/tests/profiling/should_run/all.T

@@ -106,3 +106,7 @@ test('T11627b', [ extra_run_opts('+RTS -i0 -RTS')  # census after each GC
                 ]
                 , compile_and_run
                 , [''])
+
+test('T11978a',
+     [only_ways(['profthreaded']), extra_run_opts('+RTS -hb -N10')],
+     compile_and_run, [''])