compat/snprintf: keep errno when the call succeeds

for example in t0410-partial-clone:

++ git -C repo fsck
Checking object directories: 100% (256/256), done.
Checking objects: 100% (1/1), done.
fatal: failed to read object 383670739c2f993999f3c10911ac5cb5c6591523: Result too large

when it should be

Checking object directories: 100% (256/256), done.
Checking objects: 100% (1/1), done.
dangling tag e5f4cb9fd329c512b08fb81a8e6b1f5e27658263

Author: j6t

compat/snprintf.c

@@ -22,6 +22,7 @@ int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
 	va_list cp;
 	char *s;
 	int ret = -1;
+	int save_errno = errno;
 
 	if (maxsize > 0) {
 		va_copy(cp, ap);
@@ -33,7 +34,7 @@ int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
 		str[maxsize-1] = 0;
 	}
 	if (ret != -1)
-		return ret;
+		goto out;
 
 	s = NULL;
 	if (maxsize < 128)
@@ -42,8 +43,10 @@ int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
 	while (ret == -1) {
 		maxsize *= 4;
 		str = realloc(s, maxsize);
-		if (! str)
+		if (!str) {
+			save_errno = ENOMEM;
 			break;
+		}
 		s = str;
 		va_copy(cp, ap);
 		ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, cp);
@@ -52,6 +55,8 @@ int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
 			ret = -1;
 	}
 	free(s);
+out:
+	errno = save_errno;
 	return ret;
 }