stb_sprintf: Fix string length calc

Factor out string length computation into helper func, comment
it a bit more, always use a limit to avoid 32b unsigned overflow,
and avoid reading past the bounds of non-0-terminated strings given
with specified precision.

Fixes issue #966.

stb_sprintf.h

@@ -300,6 +300,46 @@ static void stbsp__lead_sign(stbsp__uint32 fl, char *sign)
    }
 }
 
+static STBSP__ASAN stbsp__uint32 stbsp__strlen_limited(char const *s, stbsp__uint32 limit)
+{
+   char const * sn = s;
+
+   // get up to 4-byte alignment
+   for (;;) {
+      if (((stbsp__uintptr)sn & 3) == 0)
+         break;
+
+      if (!limit || *sn == 0)
+         return (stbsp__uint32)(sn - s);
+
+      ++sn;
+      --limit;
+   }
+
+   // scan over 4 bytes at a time to find terminating 0
+   // this will intentionally scan up to 3 bytes past the end of buffers,
+   // but becase it works 4B aligned, it will never cross page boundaries
+   // (hence the STBSP__ASAN markup; the over-read here is intentional
+   // and harmless)
+   while (limit >= 4) {
+      stbsp__uint32 v = *(stbsp__uint32 *)sn;
+      // bit hack to find if there's a 0 byte in there
+      if ((v - 0x01010101) & (~v) & 0x80808080UL)
+         break;
+
+      sn += 4;
+      limit -= 4;
+   }
+
+   // handle the last few characters to find actual size
+   while (limit && *sn) {
+      ++sn;
+      --limit;
+   }
+
+   return (stbsp__uint32)(sn - s);
+}
+
 STBSP__PUBLICDEF int STB_SPRINTF_DECORATE(vsprintfcb)(STBSP_SPRINTFCB *callback, void *user, char *buf, char const *fmt, va_list va)
 {
    static char hex[] = "0123456789abcdefxp";
@@ -543,37 +583,9 @@ STBSP__PUBLICDEF int STB_SPRINTF_DECORATE(vsprintfcb)(STBSP_SPRINTFCB *callback,
          s = va_arg(va, char *);
          if (s == 0)
             s = (char *)"null";
-         // get the length
-         sn = s;
-         for (;;) {
-            if ((((stbsp__uintptr)sn) & 3) == 0)
-               break;
-         lchk:
-            if (sn[0] == 0)
-               goto ld;
-            ++sn;
-         }
-         n = 0xffffffff;
-         if (pr >= 0) {
-            n = (stbsp__uint32)(sn - s);
-            if (n >= (stbsp__uint32)pr)
-               goto ld;
-            n = ((stbsp__uint32)(pr - n)) >> 2;
-         }
-         while (n) {
-            stbsp__uint32 v = *(stbsp__uint32 *)sn;
-            if ((v - 0x01010101) & (~v) & 0x80808080UL)
-               goto lchk;
-            sn += 4;
-            --n;
-         }
-         goto lchk;
-      ld:
-
-         l = (stbsp__uint32)(sn - s);
-         // clamp to precision
-         if (l > (stbsp__uint32)pr)
-            l = pr;
+         // get the length, limited to desired precision
+         // always limit to ~0u chars since our counts are 32b
+         l = stbsp__strlen_limited(s, (pr >= 0) ? pr : ~0u);
          lead[0] = 0;
          tail[0] = 0;
          pr = 0;