cpp: Avoid missing returns in non-void functions

* lang/cpp/src/context.cpp
  (Context::signaturePolicyURL): return nullptr on default
  (to_tofu_policy_t): add default case for unknown
* lang/cpp/src/key.cpp
  (Key::primaryFingerprint): return nullptr on default
* lang/cpp/src/tofuinfo.cpp
  (GpgME::TofuInfo::policy): add default case for unknown

Signed-off-by: Andreas Stieger <astieger@suse.com>
Signed-off-by: Andre Heinecke <aheinecke@intevation.de>

lang/cpp/src/context.cpp

@@ -1051,6 +1051,7 @@ const char *Context::signaturePolicyURL() const
             return n->value;
         }
     }
+    return nullptr;
 }
 
 Notation Context::signatureNotation(unsigned int idx) const
@@ -1344,6 +1345,7 @@ static gpgme_tofu_policy_t to_tofu_policy_t(unsigned int policy)
         case TofuInfo::PolicyAsk:
             return GPGME_TOFU_POLICY_ASK;
         case TofuInfo::PolicyUnknown:
+        default:
             return GPGME_TOFU_POLICY_UNKNOWN;
     }
 }

lang/cpp/src/key.cpp

@@ -273,6 +273,7 @@ const char *Key::primaryFingerprint() const
         /* Return the first subkeys fingerprint */
         return key->subkeys->fpr;
     }
+    return nullptr;
 }
 
 unsigned int Key::keyListMode() const

lang/cpp/src/tofuinfo.cpp

@@ -109,6 +109,7 @@ GpgME::TofuInfo::Policy GpgME::TofuInfo::policy() const
         case GPGME_TOFU_POLICY_ASK:
             return PolicyAsk;
         case GPGME_TOFU_POLICY_UNKNOWN:
+        default:
             return PolicyUnknown;
     }
 }