pgsql: Fix a theoretical memory leak in pg_password_sendauth().

Log Message:
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Fix a theoretical memory leak in pg_password_sendauth(). If the first
malloc() succeeded but the second failed, the buffer allocated by the
first malloc() would be leaked. Fix this by allocating both buffers
via a single malloc(), as suggested by Tom.

Per Coverity static analysis performed by EnterpriseDB.

Modified Files:
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    pgsql/src/interfaces/libpq:
        fe-auth.c (r1.102 -> r1.103)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.102&r2=1.103)