RSA: Fix buffer overflow in PSS signature verification

Fix buffer overflow in RSA-PSS signature verification when the hash is
too large for the key size. Found by Seth Terashima, Qualcomm.

Added a non-regression test and a positive test with the smallest
permitted key size for a SHA-512 hash.
diff --git a/ChangeLog b/ChangeLog
index 8f7843d..24835cd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,11 @@
 
 = mbed TLS 2.1.x released xxxx-xx-xx
 
+Security
+   * Fix buffer overflow in RSA-PSS verification when the hash is too
+     large for the key size. Found by Seth Terashima, Qualcomm Product
+     Security Initiative, Qualcomm Technologies Inc.
+
 Bugfix
    * Fix ssl_parse_record_header() to silently discard invalid DTLS records
      as recommended in RFC 6347 Section 4.1.2.7.