gh-144194: Fix mmap failure check in perf_jit_trampoline.c

mmap() returns MAP_FAILED ((void*)-1) on error, not NULL. The current
check never detects mmap failures, so jitdump initialization proceeds
even when the memory mapping fails.

Author: stratakis

Misc/NEWS.d/next/Core_and_Builtins/2026-01-23-20-20-42.gh-issue-144194.IbXfxd.rst

@@ -0,0 +1,2 @@
+Fix mmap failure check in perf jitdump initialization. The code incorrectly
+checked for NULL instead of MAP_FAILED, causing failures to go undetected.

Python/perf_jit_trampoline.c

@@ -1082,7 +1082,8 @@ static void* perf_map_jit_init(void) {
         0                        // Offset 0 (first page)
     );
 
-    if (perf_jit_map_state.mapped_buffer == NULL) {
+    if (perf_jit_map_state.mapped_buffer == MAP_FAILED) {
+        perf_jit_map_state.mapped_buffer = NULL;
         close(fd);
         return NULL;  // Memory mapping failed
     }