Fix potential out of bounds read.

The check was only done when not memory mapped, so there was a potential
out of bounds read. In addition the check only printed an error, and
didn't return and went ahead with the erronous read.

The 'loc' variable is indirectly read from the file, so in case the
history file is corrupted this could potentially lead to a crash.

Found by Coverity.

REVIEW: 128153

src/History.cpp

@@ -158,14 +158,17 @@ void HistoryFile::get(unsigned char* buffer, int size, int loc)
     if (!_fileMap && _readWriteBalance < MAP_THRESHOLD)
         map();
 
+    if (loc < 0 || size < 0 || loc + size > _length) {
+        fprintf(stderr, "getHist(...,%d,%d): invalid args.\n", size, loc);
+        return;
+    }
+
     if (_fileMap) {
         for (int i = 0; i < size; i++)
             buffer[i] = _fileMap[loc + i];
     } else {
         int rc = 0;
 
-        if (loc < 0 || size < 0 || loc + size > _length)
-            fprintf(stderr, "getHist(...,%d,%d): invalid args.\n", size, loc);
         rc = QT_LSEEK(_fd, loc, SEEK_SET);
         if (rc < 0) {
             perror("HistoryFile::get.seek");