RSA: Fix another buffer overflow in PSS signature verification

Fix buffer overflow in RSA-PSS signature verification when the masking
operation results in an all-zero buffer. This could happen at any key size.
diff --git a/tests/suites/test_suite_pkcs1_v21.data b/tests/suites/test_suite_pkcs1_v21.data
index 6d31494..7c202e9 100644
--- a/tests/suites/test_suite_pkcs1_v21.data
+++ b/tests/suites/test_suite_pkcs1_v21.data
@@ -819,3 +819,7 @@
 depends_on:MBEDTLS_SHA512_C
 pkcs1_rsassa_pss_verify_ext:521:16:"0131b69860f3cb9bf85ea358fdf2bd2990f1b77a80d6a4fdf817a43dd896bdf7dd26af8ac0237f526e0d33b105c971fdbd4ffa9ece99fc469f31ecf429e8f562c1c3":16:"010001":MBEDTLS_MD_SHA512:MBEDTLS_MD_SHA512:MBEDTLS_MD_SHA512:0:"":"00471794655837da498cbf27242807b40593a353c707eb22fd2cc5a3259e728ac4f1df676043eeec8e16c1175b3d9ac8cae72ec1d5772dd69de71c5677f19031568e":MBEDTLS_ERR_RSA_INVALID_PADDING:MBEDTLS_ERR_RSA_INVALID_PADDING
 
+RSASSA-PSS verify ext, all-zero padding, automatic salt length
+depends_on:MBEDTLS_SHA256_C
+pkcs1_rsassa_pss_verify_ext:512:16:"00b076d23250816f9aab02307e452b97f0cae7598369b41624e8afc7971a59a13892f64b07eaa6ec928c160b2d6ec8f9d0dd5b63c8b3ac0767b4f65c892f56c10f":16:"010001":MBEDTLS_MD_NONE:MBEDTLS_MD_SHA256:MBEDTLS_MD_SHA256:MBEDTLS_RSA_SALT_LEN_ANY:"":"63a35294577c7e593170378175b7df27c293dae583ec2a971426eb2d66f2af483e897bfae5dc20300a9d61a3644e08c3aee61a463690a3498901563c46041056":MBEDTLS_ERR_RSA_INVALID_PADDING:MBEDTLS_ERR_RSA_INVALID_PADDING
+