Initialize the mock options in the fuzzing code

Fixes bug 24082; bugfix on 0.3.0.3-alpha.  Found by Brian Carpenter.

changes/bug24082

+  o Minor bugfixes (testing):
+    - Fix a spurious fuzzing-only use of an uninitialized value.
+      Found by Brian Carpenter. Fixes bug 24082; bugfix on 0.3.0.3-alpha.

src/test/fuzz/fuzzing_common.c

@@ -107,7 +107,7 @@ global_init(void)
   configure_backtrace_handler(get_version());
 
   /* set up the options. */
-  mock_options = tor_malloc(sizeof(or_options_t));
+  mock_options = tor_malloc_zero(sizeof(or_options_t));
   MOCK(get_options, mock_get_options);
 
   /* Make BUG() and nonfatal asserts crash */