Check for NULL as input to extrainfo_parse_entry_from_string()

We hope this will make the clangalyzer less worried about this function.

Closes ticket 21496.

changes/bug21496

@@ -0,0 +1,4 @@
+  o Minor features (safety):
+    - Add an explict check to extrainfo_parse_entry_from_string() for NULL
+      inputs. We don't believe this can actually happen, but it may help
+      silence a warning from the Clang analyzer. Closes ticket 21496.

src/or/routerparse.c

@@ -2030,6 +2030,9 @@ extrainfo_parse_entry_from_string(const char *s, const char *end,
    * parse that's covered by the hash. */
   int can_dl_again = 0;
 
+  if (BUG(s == NULL))
+    return NULL;
+
   if (!end) {
     end = s + strlen(s);
   }