[sanitizer] Fix Fuchsia ReadBinaryName not to crash when uninitialized

[mcgrathr]

Summary

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.

compiler-rt/trunk/lib/sanitizer_common/sanitizer_fuchsia.cc

for (char **Env = StoredEnviron; *Env != 0; Env++) {
       if (internal_strncmp(*Env, name, NameLen) == 0 && (*Env)[NameLen] == '=')
         return (*Env) + NameLen + 1;
     }
   }
   return nullptr;
 }
 
 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);
 }
 
 uptr ReadLongProcessName(/*out*/ char *buf, uptr buf_len) {
   return ReadBinaryName(buf, buf_len);
 }