Look into invalid read/writes detected by Valgrind when loading aros rom/ext-rom

[FrodeSolheim]

==908642== Thread 6 emulation:
==908642== Invalid read of size 4
==908642==    at 0x5D3857: getromdatabydata(unsigned char*, int) (rommgr.cpp:1134)
==908642==    by 0x594333: scan_single_rom_2 (roms.cpp:123)
==908642==    by 0x594333: scan_single_rom (roms.cpp:167)
==908642==    by 0x594333: amiga_add_rom_file (roms.cpp:221)
==908642==    by 0x415329: fs_uae_load_rom_files (main.c:477)
==908642==    by 0x4102F7: fs_uae_configure_amiga_hardware (config.c:138)
==908642==    by 0x414831: on_init (main.c:527)
==908642==    by 0x594D5B: machdep_init() (support.cpp:15)
==908642==    by 0x52D3BF: real_main2 (main.cpp:1092)
==908642==    by 0x52D3BF: real_main(int, char**) (main.cpp:1212)
==908642==    by 0x58FB78: amiga_main (libamiga.cpp:315)
==908642==    by 0x4145EC: main_function (main.c:750)
==908642==    by 0xC0F040: emulation_thread (emu.c:413)
==908642==    by 0x7ACD3C0: g_thread_proxy (gthread.c:820)
==908642==    by 0x8468926: start_thread (pthread_create.c:435)
==908642==  Address 0x87e4d50 is 0 bytes after a block of size 0 alloc'd
==908642==    at 0x7A42A23: calloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)