Description:
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When using the readimageblob method to convert a binary string into a valid image resource, the readimageblob method results in a segmentation fault if the second file name argument is not supplied.

Reproduce code:
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Given a valid image file stored as a binary string in the variable $data, execution fails on :

$img = Gmagick();
$img->readimageblob($data);

However, execution succeeds on:

$img = Gmagick();
$img->readimageblob($data, 'dummyfilename');

where the only difference is to include a value for the second file name argument.

Expected result:
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I expected the binary string stored in $data to be processed by the method as a valid Gmagick image resource for further processing.

Actual result:
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Following notice written to apache error logs:

exit signal Segmentation fault (11)