Make stl_vsprintf return if there is an vsnprintf error

vsnprintf man page claims:
"If an output error is encountered, a negative value is returned."

That means we has to call vsnprintf twice at most to have whole output
written to a string. But in case of error we return an empty string.

The code also optimizes an expected common case of outputting single
line with a small stack allocated buffer. If the stack buffer is too
small then it uses std::string::resize to allocate exactly enough memory
and writes directly to std::string.

Second call could use vsprintf because memory is known to be large
enough. But I think that difference isn't detectable outside micro
benchmarks.

library/MiscUtils.cpp

@@ -40,6 +40,7 @@ distribution.
 
 #include <sstream>
 #include <map>
+#include <array>
 
 std::string stl_sprintf(const char *fmt, ...) {
     va_list lst;
@@ -50,21 +51,23 @@ std::string stl_sprintf(const char *fmt, ...) {
 }
 
 std::string stl_vsprintf(const char *fmt, va_list args) {
-    std::vector<char> buf;
-    buf.resize(4096);
-    for (;;) {
+    /* Allow small (about single line) strings to be printed into stack memory
+     * with a call to vsnprintf.
+     */
+    std::array<char,128> buf;
     va_list args2;
     va_copy(args2, args);
     int rsz = vsnprintf(&buf[0], buf.size(), fmt, args2);
     va_end(args2);
-
     if (rsz < 0)
-            buf.resize(buf.size()*2);
-        else if (unsigned(rsz) >= buf.size())
-            buf.resize(rsz+1);
-        else
-            return std::string(&buf[0], rsz);
-    }
+        return std::string(); /* Error occurred */
+    if (static_cast<unsigned>(rsz) < buf.size())
+        return std::string(&buf[0], rsz); /* Whole string fits to a single line buffer */
+    std::string rv;
+    // Allocate enough memory for the output and null termination
+    rv.resize(rsz+1);
+    rsz = vsnprintf(&rv[0], rv.size(), fmt, args);
+    return rv;
 }
 
 bool split_string(std::vector<std::string> *out,