Address cppcheck warnings generated after addressing Sonarcloud suggestions

- The following two warnings were generated after introducing the change
  to instantiate the DigestImpl template with the address of mbedtls_md5
  or mbedtls_sha1:
  - warning: src/utils/sha1.h,62,error,danglingTemporaryLifetime,Using
    pointer that is a temporary.
  - warning: src/utils/sha1.h,60,style,constVariablePointer,Variable
    'ret' can be declared as pointer to const
- See owasp-modsecurity#3231 (comment)

Author: eduar-hte

src/utils/sha1.h

@@ -57,7 +57,7 @@ class DigestImpl {
         ConvertOp convertOp) -> auto {
         char digest[DigestSize];
 
-        auto ret = digestOp(reinterpret_cast<const unsigned char *>(input.c_str()),
+        const auto ret = (*digestOp)(reinterpret_cast<const unsigned char *>(input.c_str()),
                  input.size(), reinterpret_cast<unsigned char *>(digest));
         assert(ret == 0);