There seems to be a missing double quote in a string in src/event/ngx_event_openssl.c. Specifically, this line:

                  "SSL_CTX_set0_tmp_dh_pkey(\%s\") failed", file->data);

should probably be this (added a double quote before the percent sign):

                  "SSL_CTX_set0_tmp_dh_pkey(\"%s\") failed", file->data);

The original string has the nonstandard escape %, and it seems quite clear by looking at other strings in the file that the intent was to write "%.