check invalid metadata

While the core issue (CONC-820) was fixed in the underlying driver, we require an additional defensive check in the Python extension layer to mitigate risks when linked against older, unpatched versions of Connector/C.

A malicious or compromised server can manipulate field metadata—specifically inflating the length of a NEWDECIMAL field. If this invalid length is passed unfiltered to the Python C-extension, the alloca() call will attempt to allocate excessive memory on the stack, leading to a stack exhaustion/overflow or stack smashing, crashing the application."