Avoid segfault on bogus SVG data

[a740g]

This PR avoids a segfault that happens when binary data pretending to be valid SVG text is passed to the SVG parser.

For example:

img& = _LOADIMAGE("mysvg.svg", 32)  ' here if mysvg.svg has invalid binary junk it will trigger the segfault

For memory loads we check the data byte by byte while copying the data for nsvgParse().
For file loads we check the file extension and check the data before passing it to nsvgParse().

[mkilgore]

@a740g do we have any sense of where in nanosvg it blows up with a seg-fault?

I think ideally we'd address the issue in there, but only if it's simple enough. If it's a more fundamental problem with nanosvg then this type of approach makes sense, but we may want to consider looking for a replacement for nanosvg that correctly validates the input 🤷‍♂️

[mkilgore]

Note that char is typically signed so the +127 unicode values would trigger the c < 32 check (I think the auto will be char due to the type of svgString). You should probably specify unsigned here if we want to allow them.

[a740g]

You are correct. Nice catch!

[a740g]

@a740g do we have any sense of where in nanosvg it blows up with a seg-fault?

I think ideally we'd address the issue in there, but only if it's simple enough. If it's a more fundamental problem with nanosvg then this type of approach makes sense, but we may want to consider looking for a replacement for nanosvg that correctly validates the input 🤷‍♂️

I am checking on this one now. It usually happens inside nsvgParse().