fix: Avoid NULL memcpy in png_set_text

[samuelgirmametaferia]

Summary

Avoid calling memcpy() with a NULL source pointer when png_set_text() receives an iTXt entry with an empty language or translated keyword.

png_text explicitly permits lang and lang_key to be NULL. The existing code computes a zero length for those fields but still passes the NULL pointer to memcpy(), which is undefined behavior even when the copy length is zero and is diagnosed by UBSan.

Guard the two copies on their computed lengths and extend pnggetset so the documented NULL iTXt language pointers are exercised.

Validation

• git diff --check: pass
• CMake build: pass
• Full CTest suite: 37/37 pass
• Pristine libpng18 with only the regression trigger: UBSan fails at pngset.c:1108 with null pointer passed as argument 2
• Patched branch under the same UBSan configuration: pnggetset passes