Avoid negative integer overflow when filesize < io_->tell().

This fixes #791.

tests/bugfixes/github/test_issue_791.py

@@ -0,0 +1,27 @@
+import system_tests
+
+
+class IntegerOverflowInWebpImageReadMetadata(
+        metaclass=system_tests.CaseMeta):
+    """
+    Regression test for the bug described in:
+    https://github.com/Exiv2/exiv2/issues/791
+
+    Due to an integer overflow bug (#791), this test triggers a 4GB
+    memory allocation. So the test will fail with a std::bad_alloc
+    exception if less than 4GB is available.  On Linux, you can use
+    `ulimit -v 4000000` to reduce the available memory to slightly
+    less than 4GB.
+    """
+    url = "https://github.com/Exiv2/exiv2/issues/791"
+
+    filename = system_tests.path(
+        "$data_path/issue_791_poc1.webp"
+    )
+    commands = ["$exiv2 $filename"]
+    stdout = [""]
+    stderr = ["""Exiv2 exception in print action for file $filename:
+corrupted image metadata
+"""
+]
+    retval = [1]