signature: use GITERR_CHECK_ALLOC to check for OOM situation

When checking for out of memory situations we usually use the
GITERR_CHECK_ALLOC macro. Besides conforming to our current code
base it adds the benefit of silencing errors in Coverity due to
Coverity handling the macro's error path as abort.

src/signature.c

@@ -79,10 +79,9 @@ int git_signature_new(git_signature **sig_out, const char *name, const char *ema
 	GITERR_CHECK_ALLOC(p);
 
 	p->name = extract_trimmed(name, strlen(name));
+	GITERR_CHECK_ALLOC(p->name);
 	p->email = extract_trimmed(email, strlen(email));
-
-	if (p->name == NULL || p->email == NULL)
-		return -1; /* oom */
+	GITERR_CHECK_ALLOC(p->email);
 
 	if (p->name[0] == '\0' || p->email[0] == '\0') {
 		git_signature_free(p);