[sanitizer] Fix Fuchsia ReadBinaryName not to crash when uninitialized

If the sanitizer runtime is loaded in a binary that doesn't really
support it, then __sanitizer_startup_hook will never have been
called to initialize StoredArgv. This case can't be supported, but
its failure mode shouldn't be to crash in sanitizer_common internals.

Patch By: mcgrathr

Differential Revision: https://reviews.llvm.org/D46344

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@331382 91177308-0d34-0410-b5e6-96231b3b80d8

Author: petrhosek

lib/sanitizer_common/sanitizer_fuchsia.cc

@@ -431,8 +431,10 @@ const char *GetEnv(const char *name) {
 }
 
 uptr ReadBinaryName(/*out*/ char *buf, uptr buf_len) {
-  const char *argv0 = StoredArgv[0];
-  if (!argv0) argv0 = "<UNKNOWN>";
+  const char *argv0 = "<UNKNOWN>";
+  if (StoredArgv && StoredArgv[0]) {
+    argv0 = StoredArgv[0];
+  }
   internal_strncpy(buf, argv0, buf_len);
   return internal_strlen(buf);
 }