Add upb_symtab_lookupfile2 (#281)

* Add upb_symtab_lookupfile2

Similar to upb_symtab_lookupfile but doesn't assume file name ends with '\0'

* Fix

upb/def.c

@@ -2042,6 +2042,13 @@ const upb_filedef *upb_symtab_lookupfile(const upb_symtab *s, const char *name)
                                                   : NULL;
 }
 
+const upb_filedef *upb_symtab_lookupfile2(
+    const upb_symtab *s, const char *name, size_t len) {
+  upb_value v;
+  return upb_strtable_lookup2(&s->files, name, len, &v) ?
+      upb_value_getconstptr(v) : NULL;
+}
+
 int upb_symtab_filecount(const upb_symtab *s) {
   return (int)upb_strtable_count(&s->files);
 }

upb/def.h

@@ -303,6 +303,8 @@ const upb_msgdef *upb_symtab_lookupmsg2(
     const upb_symtab *s, const char *sym, size_t len);
 const upb_enumdef *upb_symtab_lookupenum(const upb_symtab *s, const char *sym);
 const upb_filedef *upb_symtab_lookupfile(const upb_symtab *s, const char *name);
+const upb_filedef *upb_symtab_lookupfile2(
+    const upb_symtab *s, const char *name, size_t len);
 int upb_symtab_filecount(const upb_symtab *s);
 const upb_filedef *upb_symtab_addfile(
     upb_symtab *s, const google_protobuf_FileDescriptorProto *file,