Fix bug in SanitizerCommon.StartSubprocessTest causing flaky behavior.

[zansari]

Summary

The tests

SanitizerCommon-Unit :: Sanitizer-i386-Test/SanitizerCommon.StartSubprocessTest
SanitizerCommon-Unit :: Sanitizer-x86_64-Test/SanitizerCommon.StartSubprocessTest

can randomly fail depending on stack layout and random bits on the stack.
This is due to a string of arrays not being properly NULL terminated before being passed to "execv". From the docs:

"The execv(), execvp(), and execvpe() functions provide an array of pointers to null-terminated strings that represent the argument list available to the new program. The first argument, by convention, should point to the filename associated with the file being executed. The array of pointers must be terminated by a NULL pointer."

[kcc]

LGTM, thanks!

compiler-rt/trunk/lib/sanitizer_common/tests/sanitizer_linux_test.cc

 TEST(SanitizerCommon, StartSubprocessTest) {
   int pipe_fds[2];
   ASSERT_EQ(0, pipe(pipe_fds));
 #if SANITIZER_ANDROID
   const char *shell = "/system/bin/sh";
 #else
   const char *shell = "/bin/sh";
 #endif
-  const char *argv[] = {shell, "-c", "echo -n 'hello'"};
+  const char *argv[] = {shell, "-c", "echo -n 'hello'", (char *)NULL};
   int pid = StartSubprocess(shell, argv,
                             /* stdin */ kInvalidFd, /* stdout */ pipe_fds[1]);
   ASSERT_GT(pid, 0);
 
   // wait for process to finish.
   while (IsProcessRunning(pid)) {
   }
   ASSERT_FALSE(IsProcessRunning(pid));