curl:file2string: load large files much faster

... by using a more efficient realloc scheme.

Bug: https://curl.haxx.se/mail/lib-2019-09/0045.html
Closes #4336

src/tool_paramhlp.c

@@ -58,12 +58,17 @@ struct getout *new_getout(struct OperationConfig *config)
 
 ParameterError file2string(char **bufp, FILE *file)
 {
-  char *ptr;
   char *string = NULL;
-
   if(file) {
+    char *ptr;
+    size_t alloc = 512;
+    size_t alloc_needed;
     char buffer[256];
     size_t stringlen = 0;
+    string = malloc(alloc);
+    if(!string)
+      return PARAM_NO_MEM;
+
     while(fgets(buffer, sizeof(buffer), file)) {
       size_t buflen;
       ptr = strchr(buffer, '\r');
@@ -73,12 +78,24 @@ ParameterError file2string(char **bufp, FILE *file)
       if(ptr)
         *ptr = '\0';
       buflen = strlen(buffer);
-      ptr = realloc(string, stringlen + buflen + 1);
-      if(!ptr) {
-        Curl_safefree(string);
-        return PARAM_NO_MEM;
+      alloc_needed = stringlen + buflen + 1;
+      if(alloc < alloc_needed) {
+#if SIZEOF_SIZE_T < 8
+        if(alloc >= (size_t)SIZE_T_MAX/2) {
+          Curl_safefree(string);
+          return PARAM_NO_MEM;
+        }
+#endif
+        /* doubling is enough since the string to add is always max 256 bytes
+           and the alloc size start at 512 */
+        alloc *= 2;
+        ptr = realloc(string, alloc);
+        if(!ptr) {
+          Curl_safefree(string);
+          return PARAM_NO_MEM;
+        }
+        string = ptr;
       }
-      string = ptr;
       strcpy(string + stringlen, buffer);
       stringlen += buflen;
     }