Saleem Abdulrasool: cleanup a few more compile warnings emitted by GCC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@171173 91177308-0d34-0410-b5e6-96231b3b80d8

src/thread.cpp

@@ -67,8 +67,10 @@ thread::hardware_concurrency() _NOEXCEPT
     return n;
 #elif defined(_POSIX_C_SOURCE) && (_POSIX_C_SOURCE >= 200112L) && defined(_SC_NPROCESSORS_ONLN)
     long result = sysconf(_SC_NPROCESSORS_ONLN);
-    if (result < 0 || result > UINT_MAX)
-        result = 0;
+    // sysconf returns -1 if the name is invalid, the option does not exist or
+    // does not have a definite limit.
+    if (result == -1)
+        return 0;
     return result;
 #else  // defined(CTL_HW) && defined(HW_NCPU)
     // TODO: grovel through /proc or check cpuid on x86 and similar