Fix NULL dereference and memory leak in HTTP output plugin

[coderabbitai]

Problem

Two potential issues identified in plugins/out_http/http.c:

1. NULL dereference risk: flb_http_client() can return NULL, but the code immediately dereferences c->proxy.host without checking
2. Memory leak on early return: When http_request() returns before append_headers(), header strings allocated by extract_headers() are leaked

Location

• File: plugins/out_http/http.c
• Function: http_request() (formerly http_post())
• Lines: ~175-181 and cleanup section

Solution

• Add NULL check after flb_http_client() call
• Free header strings on early return path
• Guard client destruction with NULL check

Context

• Identified during review of PR out_http: Added PUT support #10882 (PUT support implementation)
• Issue discussion: out_http: Added PUT support #10882 (comment)
• Reporter: @nicknezis

Impact

• Severity: Medium (potential crash on client creation failure)
• Frequency: Low (depends on memory pressure/network conditions)