examples/csr: size PEM buffer for RSA-4096 signatures

MAX_PEM_SIZE aliased to MAX_CONTEXT_SIZE (2 KB), which is enough
for an RSA-2048 self-signed cert but overflows at RSA-4096 where
the signature alone is 512 B plus the cert body, ASN.1, and base64
overhead. wc_MakeCertReq / wc_MakeSelfSignedCert then returned
BUFFER_E (0xffffff7c / 0xffffff53).

Gate on MAX_RSA_KEY_BITS so 2048-bit builds keep the exact same
buffer size; 4096-bit builds (Nations NS350, Infineon SLB967x, or
anyone overriding) get 4 KB. Mirrors the fix already applied to
examples/pkcs7/pkcs7.c.

Author: aidangarske

examples/csr/csr.c

@@ -60,7 +60,14 @@ static const char* gClientCertEccFile = ECC_CERT_PEM;
 #endif
 
 #ifndef MAX_PEM_SIZE
-#define MAX_PEM_SIZE MAX_CONTEXT_SIZE
+    /* Must hold the full PEM-encoded CSR/cert (cert body + RSA signature +
+     * ASN.1 + base64 overhead). MAX_CONTEXT_SIZE (2 KB) fits RSA-2048 but
+     * overflows at RSA-4096 where the signature alone is 512 B. */
+    #if MAX_RSA_KEY_BITS >= 4096
+        #define MAX_PEM_SIZE 4096
+    #else
+        #define MAX_PEM_SIZE MAX_CONTEXT_SIZE
+    #endif
 #endif
 
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