x86: Fix out-of-bounds heap write in sample_perf_regs_mapping

x86: Fix out-of-bounds heap write in sample_perf_regs_mapping

x86_sample_perf_regs_mapping initialized perf_to_regs[] only up to the
highest set bit of perf_regs_mask, but indexed it with dwarf_to_perf[i]
values up to PERF_REG_X86_64_MAX-1. For a sample carrying only the base
GP registers the upper slots stay uninitialized, and the resulting index
j was used unchecked to write cached_regs_mapping[j], a count-element
calloc buffer. Initialize the whole perf_to_regs[] array to -1, bounds-
check j before the write, and check calloc for failure.

	* backends/x86_initreg_sample.c (x86_sample_perf_regs_mapping):
	Initialize full perf_to_regs array, bounds-check index, check
	calloc result.

Signed-off-by: Sayed Kaif <skaif@digiscrypt.com>