AArch64: Fix diet-mode NULL deref in printMatrixTileVector (#3017)

In CAPSTONE_DIET builds, getRegisterName() returns NULL because the
register-name string table is compiled out. Every other AArch64 printer
helper feeds that NULL into SStream_concat0(), which is a no-op on NULL
under CAPSTONE_DIET. printMatrixTileVector is the outlier: it calls
strlen(RegName) + 1 directly to size a scratch buffer, so a NULL
result causes SIGSEGV inside strlen().

Early-return after AArch64_add_cs_detail_1() when RegName is NULL, so
detail-mode users still get the operand recorded.

Repro on a diet aarch64 build:

    static const unsigned char code[] = { 0xc0, 0xc0, 0xc0, 0xc0 };
    cs_disasm(handle, code, sizeof(code), 0, 0, &insn);  /* SIGSEGV */

Fixes capstone-engine/capstone#3014

Co-authored-by: Amit Sides <amit-sides@users.noreply.github.com>

arch/AArch64/AArch64InstPrinter.c

@@ -964,6 +964,10 @@ DEFINE_printMatrix(0);
 \
 		const char *RegName = getRegisterName(MCOperand_getReg(RegOp), \
 						      AArch64_NoRegAltName); \
+\
+		/* Diet builds have no register-name table; skip printing. */ \
+		if (!RegName) \
+			return; \
 \
 		unsigned buf_len = strlen(RegName) + 1; \
 		char *Base = cs_mem_calloc(1, buf_len); \