Silence gcc warnings (#973)

* Silence gcc warning in bcf_hdr_printf()

For some optimisation levels, gcc would report a format-truncation
warning for the line that measured how much memory was needed to
store the header string.  Silence it by supplying a reasonably
big buffer, which has the added bonus that the malloc and second
call to vsnprintf can be avoided completely in most cases.

* Silence gcc 4 warning

Silence 'end' may be used uninitialized warning from gcc 4
by making it more obvious that EOF is returned when 'start' is
NULL (and 'end' is not used in that case).

hfile_s3_write.c

@@ -171,13 +171,12 @@ static int get_entry(char *in, char *start_tag, char *end_tag, kstring_t *out) {
     }
 
     start = strstr(in, start_tag);
+    if (!start) return EOF;
 
-    if (start) {
-        start += strlen(start_tag);
-        end = strstr(start, end_tag);
-    }
+    start += strlen(start_tag);
+    end = strstr(start, end_tag);
 
-    if (!start || !end) return EOF;
+    if (!end) return EOF;
 
     return kputsn(start, end - start, out);
 }

vcf.c

@@ -942,19 +942,26 @@ void bcf_hdr_remove(bcf_hdr_t *hdr, int type, const char *key)
 
 int bcf_hdr_printf(bcf_hdr_t *hdr, const char *fmt, ...)
 {
+    char tmp[256], *line = tmp;
     va_list ap;
     va_start(ap, fmt);
-    int n = vsnprintf(NULL, 0, fmt, ap) + 2;
+    int n = vsnprintf(line, sizeof(tmp), fmt, ap);
     va_end(ap);
 
-    char *line = (char*)malloc(n);
-    va_start(ap, fmt);
-    vsnprintf(line, n, fmt, ap);
-    va_end(ap);
+    if (n >= sizeof(tmp)) {
+        n++; // For trailing NUL
+        line = (char*)malloc(n);
+        if (!line)
+            return -1;
+
+        va_start(ap, fmt);
+        vsnprintf(line, n, fmt, ap);
+        va_end(ap);
+    }
 
     int ret = bcf_hdr_append(hdr, line);
 
-    free(line);
+    if (line != tmp) free(line);
     return ret;
 }