Fix CMock_Guts_MemBytesCapacity() integer underflow

[94xhn]

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Bug

CMock_Guts_MemBytesCapacity() (src/cmock.c) returns:

return (sizeof(CMock_Guts_Buffer) - CMOCK_MEM_ALIGN_SIZE);

CMock_Guts_Buffer is declared as static unsigned char* CMock_Guts_Buffer, so sizeof(CMock_Guts_Buffer) is the size of the pointer (8 bytes on a typical 64-bit build), not the size of the buffer it points to. Whenever CMOCK_MEM_ALIGN_SIZE > sizeof(void*) -- which happens by default on C11+ toolchains, since CMOCK_MEM_ALIGN_SIZE auto-upgrades to sizeof(max_align_t) unless the user overrides CMOCK_MEM_ALIGN (see src/cmock_internals.h) -- the subtraction underflows and CMock_Guts_MemBytesCapacity() returns a huge garbage value instead of the real capacity.

Confirmed by compiling the unmodified src/cmock.c with vendor/unity/src/unity.c (GCC, -std=c11, no CMOCK_MEM_ALIGN/CMOCK_MEM_SIZE overrides): sizeof(void*)=8, sizeof(max_align_t)=32, so CMOCK_MEM_ALIGN_SIZE=32, and CMock_Guts_MemBytesCapacity() returns 18446744073709551592 (0xffffffffffffffe8) instead of the true capacity of 32768.

This is why CI doesn't catch it: test/c/TestCMockC.yml pins CMOCK_MEM_ALIGN=2, which keeps CMOCK_MEM_ALIGN_SIZE below sizeof(void*) so no underflow occurs there, and the self-test only asserts Capacity() <= CMOCK_MEM_SIZE (an upper bound), so a wrong-but-small value still passes silently.

Fix

Use CMock_Guts_BufferSize instead, which already tracks the real buffer size in both the default/static branch and the CMOCK_MEM_DYNAMIC branch (including growth via realloc). This makes MemBytesCapacity() == MemBytesUsed() + MemBytesFree() by construction, matching the two sibling accessors that already use CMock_Guts_BufferSize/CMock_Guts_FreePtr correctly.

Testing

• Compiled the unmodified function standalone and confirmed the underflow reproduces exactly as described above.
• Applied this fix and confirmed MemBytesCapacity() == MemBytesUsed() + MemBytesFree() in the default/static config, and in CMOCK_MEM_DYNAMIC mode both before and after a realloc-driven buffer growth.
• Built and ran the existing test/c/TestCMockC.c suite with the exact CMOCK_MEM_STATIC / CMOCK_MEM_ALIGN=2 / CMOCK_MEM_INDEX_TYPE=int configuration from test/c/TestCMockC.yml: all 9 tests still pass with this fix (no regression).