libexif / libexif / 39258192975

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Share one range check and size multiply helper between the loaders (#278)

exif-data.c and the Canon, Fuji, Olympus and Pentax MakerNote loaders
each carried an identical copy of the CHECKOVERFLOW() macro. Being a
macro it evaluated its arguments more than once (one Olympus caller
passes exif_get_long() as the size) and its comparisons took whatever
mix of argument types each caller used. The four MakerNote loaders also
open-coded the same "format size times component count" overflow test
before computing the tag size.

Replace the copies with two static inline helpers in the private
exif-system.h: exif_check_overflow(), with exactly the old macro's
semantics but size_t parameters that are evaluated once, and
exif_check_mul_overflow(), which performs the same test as the old
open-coded check and only produces the product when it passes. Callers
now get one tested implementation instead of five copies.

No change in behaviour: every caller passes non-negative values of at
most 32 bits, for which the old and new expressions agree. Add unit
checks for both helpers to test-integers.

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