Fix UBSAN false positive (backport #3483)

[mergify]

This fixes a (false positive) error from UBSAN:

INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 3590403533
INFO: Loaded 2 modules   (184486 inline 8-bit counters): 183894 [0x76d8651991c0, 0x76d8651c6016), 592 [0x62495a70b448, 0x62495a70b698), 
INFO: Loaded 2 PC tables (184486 PCs): 183894 [0x76d8651c6018,0x76d865494578), 592 [0x62495a70b698,0x62495a70db98), 
./bin/fuzz-read-print-write: Running 1 inputs 1 time(s) each.
Running: crash-01c9e1813f47c45ee6be5c976f16641176e51982
/home/kev/work/exiv2/src/image.cpp:651:10: runtime error: null pointer passed as argument 1, which is declared to never be null
/usr/include/string.h:44:28: note: nonnull attribute specified here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/kev/work/exiv2/src/image.cpp:651:10 

It's a false positive because size is zero, so it doesn't matter that bytes is NULL. But it's easy to fix by returning early if size == 0.

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This is an automatic backport of pull request #3483 done by Mergify.