API contract violation in `blosc2_compcode_to_compname` on unknown compcode

[hgarrereyn]

On code>=6, blosc2_compcode_to_compname returns a non-negative code but produces a null compname; according to the docs, -1 should be returned if the codec is not recognized. (not a serious issue but could be fixed easily)

I believe the issue is here:

c-blosc2/blosc/blosc2.c

Lines 293 to 294 in 4808890

	else if (compcode >= BLOSC_LAST_CODEC)
	code = compcode;

testcase.cpp

#include <cstdio>
#include <cstdlib>
extern "C" {
#include "blosc2.h"
}

int main() {
  // Pick a compcode that is not mapped/available in this build; 6 reproduces the issue here
  int code = 6;
  const char* compname = nullptr;
  int rc = blosc2_compcode_to_compname(code, &compname);
  // According to documentation, unknown/unsupported codes should return -1.
  // Observed behavior: rc == code (>=0) and compname == NULL for code 6.
  if (rc >= 0 && compname == nullptr) {
    // Demonstrate the contract violation
    fprintf(stderr, "contract violation: rc=%d, compname is NULL for code=%d\n", rc, code);
    abort();
  }
  return 0;
}