draw: check for an integer overflow when computing stride
draw: check for an integer overflow when computing stride

Our buffer overflow arithmetic was susceptible to integer
overflows which was the buffer overflow logic to break.
Lets use the llvm overflow intrinsics to check for integer
overflows while computing the stride/needed buffer size.

Signed-off-by: Zack Rusin <6ab0df0692bc027e828ec9e4e9fbe8fa1a7842a8@vmware.com>

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