Bug 1285074 - Fix pthreads error reporting for posix js::Mutex; r=terrence

The old code assumed that various pthreads calls would set errno, and therefore
we could use perror() to print information about them. That assumption was
incorrect: pthreads calls return the error number directly and do not set errno
at all. The solution: asign the return value to errno and then call perror();

js/src/threading/posix/Mutex.cpp

@@ -4,6 +4,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#include <errno.h>
 #include <pthread.h>
 #include <stdio.h>
 
@@ -13,10 +14,15 @@
 #include "threading/posix/MutexPlatformData.h"
 
 #define TRY_CALL_PTHREADS(call, msg)            \
-    if ((call) != 0) {                          \
-        perror(msg);                            \
-        MOZ_CRASH(msg);                         \
-    }
+  {                                             \
+    int result = (call);                        \
+    if (result != 0) {                          \
+      MOZ_ASSERT(!errno);                       \
+      errno = result;                           \
+      perror(msg);                              \
+      MOZ_CRASH(msg);                           \
+    }                                           \
+  }
 
 js::Mutex::Mutex()
 {