Fix memory leaks when BN_bin2bn() fails

[ndossche]

This is observable in LibreSSL. The code is creating 3 big numbers, but if one of them failed to create, then DSA_set0_pqg() will have never executed and there are no references left to the created big numbers, leaking them in the process.

E.g.:

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x7f4edd402340 in calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
    #1 0x7f4edce9d106 in BN_new bn/bn_lib.c:75
    #2 0x7f4edce9706c in bn_bin2bn_cbs bn/bn_convert.c:151
    #3 0x7f4edce97853 in BN_bin2bn bn/bn_convert.c:206
    #4 0x5556bd923e1d in php_openssl_pkey_init_dsa_data /work/php-src/ext/openssl/openssl_backend_v1.c:142
    #5 0x5556bd92428f in php_openssl_pkey_init_dsa /work/php-src/ext/openssl/openssl_backend_v1.c:186
    #6 0x5556bd8fe079 in zif_openssl_pkey_new /work/php-src/ext/openssl/openssl.c:2042
    #7 0x5556be6b44e5 in zend_test_execute_internal /work/php-src/ext/zend_test/observer.c:306
    #8 0x5556be9dc85a in ZEND_DO_FCALL_SPEC_RETVAL_USED_HANDLER /work/php-src/Zend/zend_vm_execute.h:2154
    #9 0x5556beb3cfa5 in execute_ex /work/php-src/Zend/zend_vm_execute.h:116519
    #10 0x5556beb51ec0 in zend_execute /work/php-src/Zend/zend_vm_execute.h:121962
    #11 0x5556becb60cc in zend_execute_script /work/php-src/Zend/zend.c:1980
    #12 0x5556be6e8ecb in php_execute_script_ex /work/php-src/main/main.c:2645
    #13 0x5556be6e92db in php_execute_script /work/php-src/main/main.c:2685
    #14 0x5556becbbc37 in do_cli /work/php-src/sapi/cli/php_cli.c:951
    #15 0x5556becbe204 in main /work/php-src/sapi/cli/php_cli.c:1362
    #16 0x7f4edca671c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    #17 0x7f4edca6728a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    #18 0x5556bd809db4 in _start (/work/php-src/build-dbg-asan/sapi/cli/php+0x609db4) (BuildId: 5cc444a6a9fc1a486ea698e72366c16bd5472605)

This was found by a hybrid static-dynamic analyser that looks for inconsistent handling of error checks in bindings.