Fix memory leak and handle realloc failure

In the previous code, there was a memory leak issue where the
previously allocated memory was not freed upon a failed realloc
operation. This commit addresses the problem by releasing the old
memory before setting the pointer to NULL in case of a realloc failure.
This ensures that memory is properly managed and avoids potential
memory leaks.

modules/dnn/src/torch/THDiskFile.cpp

@@ -375,15 +375,21 @@ static long THDiskFile_readString(THFile *self, const char *format, char **str_)
     long total = TBRS_BSZ;
     long pos = 0L;
 
+    if (p == NULL)
+        THError("read error: failed to allocate buffer");
     for (;;)
     {
       if(total-pos == 0) /* we need more space! */
       {
         total += TBRS_BSZ;
-        p = (char*)THRealloc(p, total);
+        char *new_p = (char*)THRealloc(p, total);
+        if (new_p == NULL)
+        {
+          THFree(p);
+          THError("read error: failed to reallocate buffer");
+        }
+        p = new_p;
       }
-      if (p == NULL)
-        THError("read error: failed to allocate buffer");
       pos += fread(p+pos, 1, total-pos, dfself->handle);
       if (pos < total) /* eof? */
       {
@@ -409,15 +415,21 @@ static long THDiskFile_readString(THFile *self, const char *format, char **str_)
     long pos = 0L;
     long size;
 
+    if (p == NULL)
+        THError("read error: failed to allocate buffer");
     for (;;)
     {
       if(total-pos <= 1) /* we can only write '\0' in there! */
       {
         total += TBRS_BSZ;
-        p = (char*)THRealloc(p, total);
+        char *new_p = (char*)THRealloc(p, total);
+        if (new_p == NULL)
+        {
+          THFree(p);
+          THError("read error: failed to reallocate buffer");
+        }
+        p = new_p;
       }
-      if (p == NULL)
-        THError("read error: failed to allocate buffer");
       if (fgets(p+pos, total-pos, dfself->handle) == NULL) /* eof? */
       {
         if(pos == 0L)